gc: Renamed lfsr_gc -> lfsr_fs_gc, keep lfsr_fs_unck in non-gc

- lfsr_gc -> lfsr_fs_gc
- lfsr_gc_unck -> lfsr_fs_unck

lfsr_fs_unck is surprisingly still useful in non-gc builds, since we
still have ckmeta/ckdata state. These flags can still be queried with
lfsr_fs_stat and cleared with lfsr_fs_ckmeta/ckdata/lfsr_traversal_t, so
it seems useful to keep this function around.

It's also a relatively cheap function.

Though this does mean it deserves a rename. Dropping the gc prefix
hopefully makes it clearer this function is not entirely gc-specific.

And since we no longer have lfsr_gc_setflags/setsteps, it makes sense to
rename lfsr_gc back to lfsr_fs_gc, to be consistent with the other
filesystem-wide utilities.

Code changes, apparently lfsr_fs_unck costs 12 bytes:

                   code          stack          ctx
  default before: 37792           2608          620
  default after:  37804 (+0.0%)   2608 (+0.0%)  620 (+0.0%)

  gc before:      37938           2608          768
  gc after:       37940 (+0.0%)   2608 (+0.0%)  768 (+0.0%)

tests/test_attrs.toml

@@ -531,7 +531,7 @@ code = '''
     // try compacting?
     #ifdef LFS_GC
     if (COMPACT) {
-        lfsr_gc(&lfs) => 0;
+        lfsr_fs_gc(&lfs) => 0;
     }
     #endif
 
@@ -614,7 +614,7 @@ code = '''
     // try compacting?
     #ifdef LFS_GC
     if (COMPACT) {
-        lfsr_gc(&lfs) => 0;
+        lfsr_fs_gc(&lfs) => 0;
     }
     #endif
 
@@ -4119,7 +4119,7 @@ code = '''
     // try compacting?
     #ifdef LFS_GC
     if (COMPACT) {
-        lfsr_gc(&lfs) => 0;
+        lfsr_fs_gc(&lfs) => 0;
     }
     #endif