xfs_scrub: try to repair space metadata before file metadata

xfs_scrub: try to repair space metadata before file metadata

Phase 4 (metadata repairs) of xfs_scrub has suffered a mild race
condition since the beginning of its existence.  Repair functions for
higher level metadata such as directories build the new directory blocks
in an unlinked temporary file and use atomic extent swapping to commit
the corrected directory contents into the existing directory.  Atomic
extent swapping requires consistent filesystem space metadata, but phase
4 has never enforced correctness dependencies between space and file
metadata repairs.

Before the previous patch eliminated the per-AG repair lists, this error
was not often hit in testing scenarios because the allocator generally
succeeds in placing file data blocks in the same AG as the inode.  With
pool threads now able to pop file repairs from the repair list before
space repairs complete, this error became much more obvious.

Fortunately, the new phase 4 design makes it easy to try to enforce the
consistency requirements of higher level file metadata repairs.  Split
the repair list into one for space metadata and another for file
metadata.  Phase 4 will now try to fix the space metadata until it stops
making progress on that, and only then will it try to fix file metadata.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>