xen: make xen_qlock_wait() nestable

xen_qlock_wait() isn't safe for nested calls due to interrupts. A call
of xen_qlock_kick() might be ignored in case a deeper nesting level
was active right before the call of xen_poll_irq():

CPU 1:                                   CPU 2:
spin_lock(lock1)
                                         spin_lock(lock1)
                                         -> xen_qlock_wait()
                                            -> xen_clear_irq_pending()
                                            Interrupt happens
spin_unlock(lock1)
-> xen_qlock_kick(CPU 2)
spin_lock_irqsave(lock2)
                                         spin_lock_irqsave(lock2)
                                         -> xen_qlock_wait()
                                            -> xen_clear_irq_pending()
                                               clears kick for lock1
                                            -> xen_poll_irq()
spin_unlock_irq_restore(lock2)
-> xen_qlock_kick(CPU 2)
                                            wakes up
                                         spin_unlock_irq_restore(lock2)
                                         IRET
                                           resumes in xen_qlock_wait()
                                           -> xen_poll_irq()
                                           never wakes up

The solution is to disable interrupts in xen_qlock_wait() and not to
poll for the irq in case xen_qlock_wait() is called in nmi context.

Cc: stable@vger.kernel.org
Cc: Waiman.Long@hp.com
Cc: peterz@infradead.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>