Add missed barrier for pm_gen/pm_active interaction.

Add missed barrier for pm_gen/pm_active interaction.

When we issue shootdown IPIs, we first assign zero to pm_gens to
indicate the need to flush on the next context switch in case our IPI
misses the context, next we read pm_active. On context switch we set
our bit in pm_active, then we read pm_gen. It is crucial that both
threads see the memory in the program order, otherwise invalidation
thread might read pm_active bit as zero and the context switching
thread might read pm_gen as zero.

IA32 allows CPU for both reads to see zero. We must use the barriers
between write and read. The pm_active bit set is already locked, so
only the invalidation functions need it.

I never saw it in real life, or at least I do not have a good
reproduction case. I found this during code inspection when hunting
for the Xen TLB issue reported by cperciva.

Reviewed by:	alc, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D15506