libceph: msg signing callouts don't need con argument

We can use msg->con instead - at the point we sign an outgoing message
or check the signature on the incoming one, msg->con is always set.  We
wouldn't know how to sign a message without an associated session (i.e.
msg->con == NULL) and being able to sign a message using an explicitly
provided authorizer is of no use.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>