The drbd_destroy_connection() frees the "connection" so use the _safe()
iterator to prevent a use after free.

Fixes: b6f85ef9 ("drbd: Iterate over all connections")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Link: https://lore.kernel.org/r/Y3Jd5iZRbNQ9w6gm@kiliSigned-off-by: Jens Axboe <axboe@kernel.dk>