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

@@ -38,11 +38,12 @@ enum {
 };
 
 /* values for flags field */
-#define NBD_FLAG_HAS_FLAGS    (1 << 0) /* nbd-server supports flags */
-#define NBD_FLAG_READ_ONLY    (1 << 1) /* device is read-only */
-#define NBD_FLAG_SEND_FLUSH   (1 << 2) /* can flush writeback cache */
+#define NBD_FLAG_HAS_FLAGS	(1 << 0) /* nbd-server supports flags */
+#define NBD_FLAG_READ_ONLY	(1 << 1) /* device is read-only */
+#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_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 */