x86/IRQ: desc->affinity should strictly represent the requested value

x86/IRQ: desc->affinity should strictly represent the requested value

desc->arch.cpu_mask reflects the actual set of target CPUs. Don't ever
fiddle with desc->affinity itself, except to store caller requested
values. Note that assign_irq_vector() now takes a NULL incoming CPU mask
to mean "all CPUs" now, rather than just "all currently online CPUs".
This way no further affinity adjustment is needed after onlining further
CPUs.

This renders both set_native_irq_info() uses (which weren't using proper
locking anyway) redundant - drop the function altogether.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>