call_rcu: stop using mb_set/mb_read

Use a store-release when enqueuing a new call_rcu, and a load-acquire
when dequeuing; and read the tail after checking that node->next is
consistent, which is the standard message passing pattern and it is
clearer than mb_read/mb_set.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>