SMP-only barriers were missing in checkpatch.pl

Refactor code slightly to make adding more variants easier.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NJulian Calaby <julian.calaby@gmail.com>
Acked-by: NJoe Perches <joe@perches.com>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>