btrfs/318: add _require_loop

btrfs/318 uses loopback devices, but was missing a call to _require_loop
to print the correct message if CONFIG_LOOP is not set.

Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>