bmap: Moved gbmap traversal to the end

This avoids issues with the different traversal paths with an mtree vs
inline-mtree. Previously this was broken when the mtree was inlined.

This order also makes more sense if we want to check mdirs before we
consider the gstate to be trustworthy enough for gbmap traversal.

tests/test_bmap.toml

@@ -2,10 +2,6 @@
 after = ['test_btree', 'test_mtree']
 ifdef = 'LFS3_BMAP'
 
-# maximize lookahead buffer, we don't actually gc so we only get one pass
-# of the disk for these tests
-defines.LOOKAHEAD_SIZE = '(BLOCK_COUNT+8-1) / 8'
-
 # test simple set operations
 [cases.test_bmap_set_split]
 in = 'lfs3.c'