ceph: use separate class for ceph sockets' sk_lock

Use a separate class for ceph sockets to prevent lockdep confusion.
Because ceph sockets only get passed kernel pointers, there is no
dependency from sk_lock -> mmap_sem.  If we share the same class as other
sockets, lockdep detects a circular dependency from

	mmap_sem (page fault) -> fs mutex -> sk_lock -> mmap_sem

because dependencies are noted from both ceph and user contexts.  Using
a separate class prevents the sk_lock(ceph) -> mmap_sem dependency and
makes lockdep happy.
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/messenger.c

@@ -29,6 +29,10 @@ static char tag_msg = CEPH_MSGR_TAG_MSG;
 static char tag_ack = CEPH_MSGR_TAG_ACK;
 static char tag_keepalive = CEPH_MSGR_TAG_KEEPALIVE;
 
+#ifdef CONFIG_LOCKDEP
+static struct lock_class_key socket_class;
+#endif
+
 
 static void queue_con(struct ceph_connection *con);
 static void con_work(struct work_struct *);
@@ -227,6 +231,10 @@ static struct socket *ceph_tcp_connect(struct ceph_connection *con)
 	con->sock = sock;
 	sock->sk->sk_allocation = GFP_NOFS;
 
+#ifdef CONFIG_LOCKDEP
+	lockdep_set_class(&sock->sk->sk_lock, &socket_class);
+#endif
+
 	set_sock_callbacks(sock, con);
 
 	dout("connect %s\n", pr_addr(&con->peer_addr.in_addr));