kernel: backport af_vsock lockdep cleanliness patch from v4.7-rc6

From torvalds/linux@4192f672fa which was
part of torvalds/linux@v4.7-rc6. The change is purely to ensure that vsock
is lockdep-clean.

Signed-off-by: David Sheets <dsheets@docker.com>

alpine/kernel/patches/0041-VSOCK-make-listener-child-lock-ordering-explicit.patch

@@ -0,0 +1,59 @@
+From 4192f672fae559f32d82de72a677701853cc98a7 Mon Sep 17 00:00:00 2001
+From: Stefan Hajnoczi <stefanha@redhat.com>
+Date: Thu, 23 Jun 2016 16:28:58 +0100
+Subject: [PATCH] vsock: make listener child lock ordering explicit
+
+There are several places where the listener and pending or accept queue
+child sockets are accessed at the same time.  Lockdep is unhappy that
+two locks from the same class are held.
+
+Tell lockdep that it is safe and document the lock ordering.
+
+Originally Claudio Imbrenda <imbrenda@linux.vnet.ibm.com> sent a similar
+patch asking whether this is safe.  I have audited the code and also
+covered the vsock_pending_work() function.
+
+Suggested-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
+Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+ net/vmw_vsock/af_vsock.c | 12 ++++++++++--
+ 1 file changed, 10 insertions(+), 2 deletions(-)
+
+diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
+index b5f1221..b96ac918 100644
+--- a/net/vmw_vsock/af_vsock.c
++++ b/net/vmw_vsock/af_vsock.c
+@@ -61,6 +61,14 @@
+  * function will also cleanup rejected sockets, those that reach the connected
+  * state but leave it before they have been accepted.
+  *
++ * - Lock ordering for pending or accept queue sockets is:
++ *
++ *     lock_sock(listener);
++ *     lock_sock_nested(pending, SINGLE_DEPTH_NESTING);
++ *
++ * Using explicit nested locking keeps lockdep happy since normally only one
++ * lock of a given class may be taken at a time.
++ *
+  * - Sockets created by user action will be cleaned up when the user process
+  * calls close(2), causing our release implementation to be called. Our release
+  * implementation will perform some cleanup then drop the last reference so our
+@@ -443,7 +451,7 @@ void vsock_pending_work(struct work_struct *work)
+ 	cleanup = true;
+ 
+ 	lock_sock(listener);
+-	lock_sock(sk);
++	lock_sock_nested(sk, SINGLE_DEPTH_NESTING);
+ 
+ 	if (vsock_is_pending(sk)) {
+ 		vsock_remove_pending(listener, sk);
+@@ -1292,7 +1300,7 @@ static int vsock_accept(struct socket *sock, struct socket *newsock, int flags)
+ 	if (connected) {
+ 		listener->sk_ack_backlog--;
+ 
+-		lock_sock(connected);
++		lock_sock_nested(connected, SINGLE_DEPTH_NESTING);
+ 		vconnected = vsock_sk(connected);
+ 
+ 		/* If the listener socket has received an error, then we should