drbd: Move some functions to where they are used

Move drbd_update_congested() to drbd_main.c, and drbd_req_new() and
drbd_req_free() to drbd_req.c: those functions are not used anywhere
else.

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