irq_poll: Fix irq_poll_sched()

The IRQ_POLL_F_SCHED bit is set as long as polling is ongoing.
This means that irq_poll_sched() must proceed if this bit has
not yet been set.

Fixes: commit ea51190c ("irq_poll: fold irq_poll_sched_prep into irq_poll_sched").
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>