nbd: don't set the device size until we're connected

A user reported a regression with using the normal ioctl interface on
newer kernels.  This happens because I was setting the device size
before the device was actually connected, which caused us to error out
and close everything down.  This didn't happen on netlink because we
hold the device lock the whole time we're setting things up, but we
don't do that for the ioctl path.  This fixes the problem.

Cc: stable@vger.kernel.org
Fixes: 29eaadc0 ("nbd: stop using the bdev everywhere")
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>