Remove workaround for unimmunized rcu_dereference from mce_log()

Remove the rmb() from mce_log(), since the immunized version of
rcu_dereference() makes it unnecessary.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>