libceph: ceph_build_auth() doesn't need ceph_auth_build_hello()

A static bug finder (EBA) on Linux 4.7:

    Double lock in net/ceph/auth.c
    second lock at 108: mutex_lock(& ac->mutex); [ceph_auth_build_hello]
    after calling from 263: ret = ceph_auth_build_hello(ac, msg_buf, msg_len);
    if ! ac->protocol -> true at 262
    first lock at 261: mutex_lock(& ac->mutex); [ceph_build_auth]

ceph_auth_build_hello() is never called, because the protocol is always
initialized, whether we are checking existing tickets (in delayed_work())
or getting new ones after invalidation (in invalidate_authorizer()).
Reported-by: Iago Abal <iari@itu.dk>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/auth.c

@@ -262,9 +262,7 @@ int ceph_build_auth(struct ceph_auth_client *ac,
 	int ret = 0;
 
 	mutex_lock(&ac->mutex);
-	if (!ac->protocol)
-		ret = ceph_auth_build_hello(ac, msg_buf, msg_len);
-	else if (ac->ops->should_authenticate(ac))
+	if (ac->ops->should_authenticate(ac))
 		ret = ceph_build_auth_request(ac, msg_buf, msg_len);
 	mutex_unlock(&ac->mutex);
 	return ret;