bmap: Added error propagation to ckpoints and cleaned up test TODOs

The main change is error propagation in lfs3_alloc_ckpoint. Since
lfs3_alloc_ckpoint writes to disk during bmap rebuilds, it can now fail
in all sorts of ways. Fortunately lfs3_alloc_ckpoint should only ever be
called by write operations, where these errors are be expected.

With bmap rebuild errors now reported correctly, this unblocks most of
the remaining test TODOs:

- Passing test_badblocks
- Passing test_ck
- Passing test_trvs

With this, LFS3_YES_BMAP is now passing all but two tests, which are
still ifndef-disabled as a temporary measure:

- test_btree - We make some low-level assumptions about the lookahead
  allocator when testing btrees. It's probably not worth trying to get
  this passing with the bmap allocator.

- test_grow - This one does need fixing! We currently don't update
  on-disk bmaps correctly when growing the filesystem.

Code changes minimal:

                code          stack          ctx
  before:      36912           2368          684
  after:       36912 (+0.0%)   2368 (+0.0%)  684 (+0.0%)

                code          stack          ctx
  bmap before: 38456           2400          812
  bmap after:  38512 (+0.1%)   2400 (+0.0%)  812 (+0.0%)

tests/test_grow.toml

@@ -8,7 +8,7 @@ after = [
     'test_mount',
 ]
 
-# TODO bmap workaround?
+# TODO!! lfs3_fs_grow needs to be able to adjust an on-disk bmaps
 ifndef = 'LFS3_BMAP'