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>

net/ceph/messenger.c

@@ -2677,7 +2677,7 @@ more:
 		if (ret <= 0) {
 			switch (ret) {
 			case -EBADMSG:
-				con->error_msg = "bad crc";
+				con->error_msg = "bad crc/signature";
 				/* fall through */
 			case -EBADE:
 				ret = -EIO;