powerpc/irq: Don't open code irq_soft_mask helpers

Use READ_ONCE() and WRITE_ONCE() instead of open coding
read and write of local PACA irq_soft_mask.

For the write, add a barrier to keep the memory clobber
that was there previously.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e2454434992cc932a5a34b695ae981c0b2f4c28e.1652862729.git.christophe.leroy@csgroup.eu