powerpc/mm: Make switch_mm_irqs_off() out of line

It's too big to be inline, there is no reason to keep it
that way.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Rework to incorporate the comment changes via fixes branch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>