xen: privcmd: Don't mark eventfd waiter as EXCLUSIVE

xen: privcmd: Don't mark eventfd waiter as EXCLUSIVE

Don't set WQ_FLAG_EXCLUSIVE when adding an irqfd to a wait queue, as
irqfd_wakeup() unconditionally returns '0', i.e. doesn't actually operate
in exclusive mode.

Note, the use of WQ_FLAG_PRIORITY is also dubious, but that's a problem
for another day.

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250522235223.3178519-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>