xfs_repair: fix bad next_unlinked field

xfs_repair: fix bad next_unlinked field

As of xfsprogs-4.17 we started testing whether the di_next_unlinked field
on an inode is valid in the inode verifiers. However, this field is never
tested or repaired during inode processing.

So if, for example, we had a completely zeroed-out inode, we'd detect and
fix the broken magic and version, but the invalid di_next_unlinked field
would not be touched, fail the write verifier, and prevent the inode from
being properly repaired or even written out.

Fix this by checking the di_next_unlinked inode field for validity and
clearing it if it is invalid.

Reported-by: John Jore <john@jore.no>
Fixes: 2949b4677 ("xfs: don't accept inode buffers with suspicious unlinked chains")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>