[LINUX] Fix IRQ SMP affinity logic for event channels.

[LINUX] Fix IRQ SMP affinity logic for event channels.
The logic now mimics native x86 behaviour: a request to change
affinity via /proc is held until the next interrupt on that
event channel. So /proc/irq/n/smp_affinity may not change
immediately!
Other notes:
 1. CPU-specific interrupts silently ignore requests to change
    affinity. For example, resched0, timer0, callfunc0, ...
 2. Reading smp_affinity always returns a cpumask containing
    a single cpu. An event channel can only be bound to a single
    cpu at a time. Neither Xen nor XenLinux implement IRQ
    balancing: requires a user-space balancing daemon.

Signed-off-by: Keir Fraser <keir@xensource.com>