xfs: test readlink/readdir with malformed inode mode

In addition to testing xfs_repair on inodes with malformed mode,
and fstat of those inodes on a mounted fs, try to also list content
of mock directory and readlink of mock symlink.

Readdir of mock directory triggers XFS assertion on kernel 4.9 with
XFS_DEBUG=y

XFS: Assertion failed:
     rval == 0 || args->dp->i_d.di_size == args->geo->blksize,
file: /home/amir/build/src/linux/fs/xfs/libxfs/xfs_dir2.c, line: 634

A fix patch, as suggested by Darrick, changes this ASSERT() to
return -EFSCORRUPTED ("xfs: sanity check directory inode di_size").

Merging this test should be deferred to after fix patch is merged.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>