fs: add new vfs_poll and file_can_poll helpers

These abstract out calls to the poll method in preparation for changes
in how we poll.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

virt/kvm/eventfd.c

@@ -397,7 +397,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 	 * Check if there was an event already pending on the eventfd
 	 * before we registered, and trigger it as if we didn't miss it.
 	 */
-	events = f.file->f_op->poll(f.file, &irqfd->pt);
+	events = vfs_poll(f.file, &irqfd->pt);
 
 	if (events & EPOLLIN)
 		schedule_work(&irqfd->inject);