xfs_repair: detect invalid zero-sized symlink inodes

xfs_repair: detect invalid zero-sized symlink inodes

Mathias Troiden reproduced a filesystem corruption that resulted in
a zero-sized local format symlink inode. This is invalid state and
results in an inode that cannot be accessed or modified.

The kernel detects this problem on inode access, fails and warns the
user to umount and run xfs_repair. Unfortunately, xfs_repair doesn't
even detect the problem. Thus the user has no path to recovery.

Update xfs_repair to check for invalid zero-sized symlinks and flag
them as corrupted. This results in tossing the inode, but returns
the fs to a valid state.

Reported-by: Mathias Troiden <mathias.troiden@gmail.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>