sched/membarrier: reduce the ability to hammer on sys_membarrier

In the Linux kernel, the following vulnerability has been resolved:

sched/membarrier: reduce the ability to hammer on sys_membarrier

On some systems, sys_membarrier can be very expensive, causing overall

slowdowns for everything. So put a lock on the path in order to

serialize the accesses to prevent the ability for this to be called at

too high of a frequency and saturate the machine.