MIPS: Quit using undefined behavior of ADDU in 64-bit atomic operations.

For 64-bit, we must use DADDU and DSUBU.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1483/


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