ceph: drop messages on unregistered mds sessions; cleanup

Verify the mds session is currently registered before handling
incoming messages.  Clean up message handlers to pull mds out
of session->s_mds instead of less trustworthy src field.

Clean up con_{get,put} debug output.

Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/caps.c

@@ -2600,7 +2600,7 @@ void ceph_handle_caps(struct ceph_mds_session *session,
 	struct inode *inode;
 	struct ceph_cap *cap;
 	struct ceph_mds_caps *h;
-	int mds = le64_to_cpu(msg->hdr.src.name.num);
+	int mds = session->s_mds;
 	int op;
 	u32 seq;
 	struct ceph_vino vino;