nbd: put socket in error cases

When adding a new socket we look it up and then try to add it to our
configuration.  If any of those steps fail we need to make sure we put
the socket so we don't leak them.

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