mptcp: properly annotate nested lock

MPTCP closes the subflows while holding the msk-level lock.
While acquiring the subflow socket lock we need to use the
correct nested annotation, or we can hit a lockdep splat
at runtime.
Reported-and-tested-by: Geliang Tang <geliangtang@gmail.com>
Fixes: e16163b6 ("mptcp: refactor shutdown and close")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -2119,7 +2119,7 @@ void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 
 	list_del(&subflow->node);
 
-	lock_sock(ssk);
+	lock_sock_nested(ssk, SINGLE_DEPTH_NESTING);
 
 	/* if we are invoked by the msk cleanup code, the subflow is
 	 * already orphaned