drivers/block/drbd/drbd_main.c: fix error path

Failure to create drbd_ee_mempool appears not to get checked.  Looks like
a copy-and-paste problem to me.

Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Cc: Lars Ellenberg <drbd-dev@lists.linbit.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>