x86/IRQ: fix create_irq() after c/s 24068:6928172f7ded

x86/IRQ: fix create_irq() after c/s 24068:6928172f7ded

init_one_irq_desc() must be called with interrupts enabled (as it may
call functions from the xmalloc() group). Rather than mis-using
vector_lock to also protect the finding of an unused IRQ, make this
lockless through using cmpxchg(), and obtain the lock only around the
actual assignment of the vector.

Also fold find_unassigned_irq() into its only caller.

It is, btw, questionable whether create_irq() calling
__assign_irq_vector() (rather than assign_irq_vector()) is actually
correct - desc->affinity appears to not get initialized properly in
this case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>