drbd: refactor use of first_peer_device()

Reduce the number of calls to first_peer_device(). Instead, call
first_peer_device() just once to assign a local variable peer_device.

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

diff --cc drivers/block/drbd/drbd_nl.c
Simple merge

diff --cc drivers/block/drbd/drbd_receiver.c
Simple merge

diff --cc drivers/block/drbd/drbd_req.c
Simple merge

diff --cc drivers/block/drbd/drbd_state.c
Simple merge

diff --cc drivers/block/drbd/drbd_worker.c
Simple merge