SCTP: lock_sock_nested in sctp_sock_migrate

sctp_sock_migrate() grabs the socket lock on a newly allocated socket while
holding the socket lock on an old socket.  lockdep worries that this might
be a recursive lock attempt.

 task/3026 is trying to acquire lock:
  (sk_lock-AF_INET){--..}, at: [<ffffffff88105b8c>] sctp_sock_migrate+0x2e3/0x327 [sctp]
 but task is already holding lock:
  (sk_lock-AF_INET){--..}, at: [<ffffffff8810891f>] sctp_accept+0xdf/0x1e3 [sctp]

This patch tells lockdep that this locking is safe by using
lock_sock_nested().
Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/socket.c

@@ -6123,8 +6123,11 @@ static void sctp_sock_migrate(struct sock *oldsk, struct sock *newsk,
 	 * queued to the backlog.  This prevents a potential race between
 	 * backlog processing on the old socket and new-packet processing
 	 * on the new socket.
+	 *
+	 * The caller has just allocated newsk so we can guarantee that other
+	 * paths won't try to lock it and then oldsk.
 	 */
-	sctp_lock_sock(newsk);
+	lock_sock_nested(newsk, SINGLE_DEPTH_NESTING);
 	sctp_assoc_migrate(assoc, newsk);
 
 	/* If the association on the newsk is already closed before accept()