blk-mq: enable polling support by default

Now that applications need to explicitly ask for polling we can enable it
by default in blk-mq drivers.  Note that this will only have an affect
on driver that supply a poll function, which currently only includes nvme.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stephen Bates <stephen.bates@pmcs.com>
Tested-by: Stephen Bates <stephen.bates@pmcs.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/blkdev.h

@@ -499,7 +499,8 @@ struct request_queue {
 
 #define QUEUE_FLAG_MQ_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
 				 (1 << QUEUE_FLAG_STACKABLE)	|	\
-				 (1 << QUEUE_FLAG_SAME_COMP))
+				 (1 << QUEUE_FLAG_SAME_COMP)	|	\
+				 (1 << QUEUE_FLAG_POLL))
 
 static inline void queue_lockdep_assert_held(struct request_queue *q)
 {