block: drbd: remove a stray unlock in __drbd_send_protocol()

There are two callers of this function and they both unlock the mutex so
this ends up being a double unlock.

Fixes: 44ed167d

 ("drbd: rcu_read_lock() and rcu_dereference() for tconn->net_conf")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>