nbd: update size when connected

I messed up changing the size of an NBD device while it was connected by
not actually updating the device or doing the uevent.  Fix this by
updating everything if we're connected and we change the size.

cc: stable@vger.kernel.org
Fixes: 639812a1 ("nbd: don't set the device size until we're connected")
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>