drbd: Avoid inconsistent locking warning

request_timer_fn() takes resource->req_lock via the device and releases it via
the connection.  Avoid this as it is confusing static code checkers.

Reported-by: "Dan Carpenter" <dan.carpenter@oracle.com>
Signed-off-by: Andreas Gruenbacher <agruen@linbit.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>