drbd: fix null-pointer dereference on local read error

In the Linux kernel, the following vulnerability has been resolved:

drbd: fix null-pointer dereference on local read error

In drbd_request_endio(), READ_COMPLETED_WITH_ERROR is passed to

__req_mod() with a NULL peer_device:

__req_mod(req, what, NULL, &m);

The READ_COMPLETED_WITH_ERROR handler then unconditionally passes this

NULL peer_device to drbd_set_out_of_sync(), which dereferences it,

causing a null-pointer dereference.

Fix this by obtaining the peer_device via first_peer_device(device),

matching how drbd_req_destroy() handles the same situation.