drbd: Wrong use of RCU in receive_protocol()

It is not enough to grab net_conf->integrity_alg under rcu_read_lock()
and access it outside of it; the entire net_conf object may be gone by
then.

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