UPSTREAM: fix "sched/membarrier: Fix p->mm->membarrier_state racy load"

Upstream patch fix from this thread:
https://lore.kernel.org/lkml/20191001071921.GJ4519@hirez.programming.kicks-ass.net/



Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Change-Id: Ia54bf5a29eba3c6d36e7a0b32442af7ff09a3998