nbd: add a reconfigure netlink command

We want to be able to reconnect dead connections to existing block
devices, so add a reconfigure netlink command.  We will also allow users
to change their timeout on the fly, but everything else will require a
disconnect and reconnect.  You won't be able to add more connections
either, simply replace dead connections with new more lively
connections.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/uapi/linux/nbd-netlink.h

@@ -62,6 +62,7 @@ enum {
 	NBD_CMD_UNSPEC,
 	NBD_CMD_CONNECT,
 	NBD_CMD_DISCONNECT,
+	NBD_CMD_RECONFIGURE,
 	__NBD_CMD_MAX,
 };
 #define NBD_CMD_MAX	(__NBD_CMD_MAX - 1)