xfs_repair: complain about free space only seen by one btree

xfs_repair: complain about free space only seen by one btree

During the free space btree walk, scan_allocbt claims in a comment that
we'll catch FREE1 blocks (i.e. blocks that were seen by only one free
space btree) later.  This never happens, with the result that xfs_repair
in dry-run mode can return 0 on a filesystem with corrupt free space
btrees.

Found by fuzzing xfs/358 with numrecs = middlebit (or sub).

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>