xfs: sanitize remount options

Perform basic sanitization of remount options by
passing the option string and a dummy mount structure
through xfs_parseargs and returning the result.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>