alloc: Merged LOOKAHEAD+LOOKGBMAP -> single LOOKAHEAD flag

Our flag space is already really packed, and I'm not sure having these
as separate flags is meaningful or useful for users. They both indicate
to repopulate allocators, and most users probably won't care that there
are two subtly different allocators operating under the hood.

There's an argument that LOOKAHEAD not touching disk is a useful
distinction, but in practice you really only need LOOKAHEAD work when
mounted RDWR.

So, merged the behaviors of LOOKAHEAD + LOOKGBMAP such that
LFS3_*_LOOKAHEAD requests repopulation of all allocators based on
gc_lookahead_thresh and gc_lookgbmap_thresh.

In priority order (some notes below):

1. If max(lookahead, gbmap) < gc_lookahead_thresh => repop lookahead
2. If gbmap < gc_lookgbmap_thresh                 => repop gbmap

As a plus, this makes it easier to avoid LFS3_IFDEF_GBMAP mess.

---

It's interesting to note LFS3_*_LOOKAHEAD will still repopulate the
lookahead buffer when the gbmap is present, but only if this would gain
more knowledge than was is currently in the gbmap.

I considered disabling lookahead scans completely when we have a gbmap,
but repopulating the lookahead buffer is still useful if the gbmap is at
risk of exhaustion. This is what gc_lookahead_thresh is for anyways, and
users can set gc_lookahead_thresh=0 if they want to disable this
behavior.

Relatedly, lookahead scans are actually prioritized over gbmap scans
(when they would gain knowledge). In theory this minimizes gc latency,
as gbmap scans risk triggering a full lookahead scan when building the
new gbmap.

---

Code changes minimal:

                 code          stack          ctx
  before:       35152           2136          660
  after:        35152 (+0.0%)   2136 (+0.0%)  660 (+0.0%)

                 code          stack          ctx
  gbmap before: 38076           2136          776
  gbmap after:  38080 (+0.0%)   2136 (+0.0%)  776 (+0.0%)

scripts/dbgflags.py

@@ -83,7 +83,6 @@ F_CKDATACKSUMS  = 0x01000000  # y-  Check data checksums on reads
 
 F_MKCONSISTENT  = 0x00000800  # y-  Make the filesystem consistent
 F_LOOKAHEAD     = 0x00001000  # y-  Repopulate lookahead buffer
-F_LOOKGBMAP     = 0x00002000  # y-  Repopulate the gbmap
 F_COMPACTMETA   = 0x00008000  # y-  Compact metadata logs
 F_CKMETA        = 0x00010000  # y-  Check metadata checksums
 F_CKDATA        = 0x00020000  # y-  Check metadata + data checksums
@@ -103,7 +102,6 @@ M_CKDATACKSUMS  = 0x01000000  # y-  Check data checksums on reads
 
 M_MKCONSISTENT  = 0x00000800  # y-  Make the filesystem consistent
 M_LOOKAHEAD     = 0x00001000  # y-  Repopulate lookahead buffer
-M_LOOKGBMAP     = 0x00002000  # y-  Repopulate the gbmap
 M_COMPACTMETA   = 0x00008000  # y-  Compact metadata logs
 M_CKMETA        = 0x00010000  # y-  Check metadata checksums
 M_CKDATA        = 0x00020000  # y-  Check metadata + data checksums
@@ -111,7 +109,6 @@ M_CKDATA        = 0x00020000  # y-  Check metadata + data checksums
 # File/filesystem check flags
 CK_MKCONSISTENT = 0x00000800  # --  Make the filesystem consistent
 CK_LOOKAHEAD    = 0x00001000  # --  Repopulate lookahead buffer
-CK_LOOKGBMAP    = 0x00002000  # --  Repopulate the gbmap
 CK_COMPACTMETA  = 0x00008000  # --  Compact metadata logs
 CK_CKMETA       = 0x00010000  # --  Check metadata checksums
 CK_CKDATA       = 0x00020000  # --  Check metadata + data checksums
@@ -119,7 +116,6 @@ CK_CKDATA       = 0x00020000  # --  Check metadata + data checksums
 # GC flags
 GC_MKCONSISTENT = 0x00000800  # --  Make the filesystem consistent
 GC_LOOKAHEAD    = 0x00001000  # --  Repopulate lookahead buffer
-GC_LOOKGBMAP    = 0x00002000  # --  Repopulate the gbmap
 GC_COMPACTMETA  = 0x00008000  # --  Compact metadata logs
 GC_CKMETA       = 0x00010000  # --  Check metadata checksums
 GC_CKDATA       = 0x00020000  # --  Check metadata + data checksums
@@ -139,7 +135,6 @@ I_CKDATACKSUMS  = 0x01000000  # --  Mounted with LFS3_M_CKDATACKSUMS
 
 I_MKCONSISTENT  = 0x00000800  # --  Filesystem needs mkconsistent to write
 I_LOOKAHEAD     = 0x00001000  # --  Lookahead buffer is not full
-I_LOOKGBMAP     = 0x00002000  # --  The gbmap is not full
 I_COMPACTMETA   = 0x00008000  # --  Filesystem may have uncompacted metadata
 I_CKMETA        = 0x00010000  # --  Metadata checksums not checked recently
 I_CKDATA        = 0x00020000  # --  Data checksums not checked recently
@@ -152,7 +147,6 @@ T_MTREEONLY     = 0x00000002  # --  Only traverse the mtree
 T_EXCL          = 0x00000008  # --  Error if filesystem modified
 T_MKCONSISTENT  = 0x00000800  # --  Make the filesystem consistent
 T_LOOKAHEAD     = 0x00001000  # --  Repopulate lookahead buffer
-T_LOOKGBMAP     = 0x00002000  # --  Repopulate the gbmap
 T_COMPACTMETA   = 0x00008000  # --  Compact metadata logs
 T_CKMETA        = 0x00010000  # --  Check metadata checksums
 T_CKDATA        = 0x00020000  # --  Check metadata + data checksums