arch,metag: Convert smp_mb__*()

Implement the new barriers; as per the old versions the metag atomic
imply a full barrier.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/n/tip-dqnyo215kq38wi4xcxnbpjw3@git.kernel.org


Cc: James Hogan <james.hogan@imgtec.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-metag@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>