generic/607: don't break on filesystems that don't support FSGETXATTR on dirs

generic/607: don't break on filesystems that don't support FSGETXATTR on dirs

This test requires that the filesystem support calling FSGETXATTR on
regular files and directories to make sure the FS_XFLAG_DAX flag works.
The _require_xfs_io_command tests a regular file but doesn't check
directories, so generic/607 should do that itself.  Also fix some typos.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>