cpumask: remove unused mask field from struct irqaction.

Up until 1.1.83, the primitive human tribes used struct sigaction for
interrupts.  The sa_mask field was overloaded to hold a pointer to the
name.

When someone created the new "struct irqaction" they carried across
the "mask" field as a kind of ancestor worship: the fact that it was
unused makes clear its spiritual significance.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>