drbd: Replace and remove old primitives

Centralize sock->mutex locking and unlocking in [drbd|conn]_prepare_command()
and [drbd|conn]_send_comman().

Therefore all *_send_* functions are touched to use these primitives instead
of drbd_get_data_sock()/drbd_put_data_sock() and former helper functions.

That change makes the *_send_* functions more standardized.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>