nbd: add multi-connection support

NBD can become contended on its single connection.  We have to serialize all
writes and we can only process one read response at a time.  Fix this by
allowing userspace to provide multiple connections to a single nbd device.  This
coupled with block-mq drastically increases performance in multi-process cases.
Thanks,
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/uapi/linux/nbd.h

@@ -43,6 +43,7 @@ enum {
 #define NBD_FLAG_SEND_FLUSH	(1 << 2) /* can flush writeback cache */
 /* there is a gap here to match userspace */
 #define NBD_FLAG_SEND_TRIM	(1 << 5) /* send trim/discard */
+#define NBD_FLAG_CAN_MULTI_CONN	(1 << 7)	/* Server supports multiple connections per export. */
 
 /* userspace doesn't need the nbd_device structure */