block: Export blk_poll

The new NVMe over fabrics target will make use of this outside from a
module.
Signed-off-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NSteve Wise <swise@opengridcomputing.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

block/blk-core.c

@@ -3385,6 +3385,7 @@ bool blk_poll(struct request_queue *q, blk_qc_t cookie)
 
 	return false;
 }
+EXPORT_SYMBOL_GPL(blk_poll);
 
 #ifdef CONFIG_PM
 /**