MIPS: Add nudges to writes for bit unlocks.

Flushing the writes lets other CPUs waiting for the lock to get it sooner.

Signed-off-by: Chad Reese <kreese@caviumnetworks.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17289/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>