xfs: don't special case shared superblock mounts

Neither kernel or userspace support shared read-only mounts, so
don't bother special casing the support check to be different
between kernel and userspace. The same check can be used as neither
like it...

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>