libceph: add nocephx_sign_messages option

Support for message signing was merged into 3.19, along with
nocephx_require_signatures option.  But, all that option does is allow
the kernel client to talk to clusters that don't support MSG_AUTH
feature bit.  That's pretty useless, given that it's been supported
since bobtail.

Meanwhile, if one disables message signing on the server side with
"cephx sign messages = false", it becomes impossible to use the kernel
client since it expects messages to be signed if MSG_AUTH was
negotiated.  Add nocephx_sign_messages option to support this use case.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/net/ceph/auth_x.c b/net/ceph/auth_x.c
index 3a544ca..10d87753 100644
--- a/net/ceph/auth_x.c
+++ b/net/ceph/auth_x.c
@@ -8,6 +8,7 @@
 
 #include <linux/ceph/decode.h>
 #include <linux/ceph/auth.h>
+#include <linux/ceph/libceph.h>
 #include <linux/ceph/messenger.h>
 
 #include "crypto.h"
@@ -698,6 +699,9 @@
 {
 	int ret;
 
+	if (ceph_test_opt(from_msgr(msg->con->msgr), NOMSGSIGN))
+		return 0;
+
 	ret = calcu_signature((struct ceph_x_authorizer *)auth->authorizer,
 			      msg, &msg->footer.sig);
 	if (ret < 0)
@@ -712,6 +716,9 @@
 	__le64 sig_check;
 	int ret;
 
+	if (ceph_test_opt(from_msgr(msg->con->msgr), NOMSGSIGN))
+		return 0;
+
 	ret = calcu_signature((struct ceph_x_authorizer *)auth->authorizer,
 			      msg, &sig_check);
 	if (ret < 0)