xfs: sanity check directory inode di_size

xfs: sanity check directory inode di_size

This changes fixes an assertion hit when fuzzing on-disk
i_mode values.

The easy case to fix is when changing an empty file
i_mode to S_IFDIR. In this case, xfs_dinode_verify()
detects an illegal zero size for directory and fails
to load the inode structure from disk.

For the case of non empty file whose i_mode is changed
to S_IFDIR, the ASSERT() statement in xfs_dir2_isblock()
is replaced with return -EFSCORRUPTED, to avoid interacting
with corrupted jusk also when XFS_DEBUG is disabled.

Suggested-by: Darrick J. Wong <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Amir Goldstein <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>