UPSTREAM: af_unix: Guard against other == sk in unix_dgram_sendmsg

(cherry picked from commit a5527dda)

The unix_dgram_sendmsg routine use the following test

if (unlikely(unix_peer(other) != sk && unix_recvq_full(other))) {

to determine if sk and other are in an n:1 association (either
established via connect or by using sendto to send messages to an
unrelated socket identified by address). This isn't correct as the
specified address could have been bound to the sending socket itself or
because this socket could have been connected to itself by the time of
the unix_peer_get but disconnected before the unix_state_lock(other). In
both cases, the if-block would be entered despite other == sk which
might either block the sender unintentionally or lead to trying to unlock
the same spin lock twice for a non-blocking send. Add a other != sk
check to guard against this.

Fixes: 7d267278 ("unix: avoid use-after-free in ep_remove_wait_queue")
Reported-By: Philipp Hahn <pmhahn@pmhahn.de>
Signed-off-by: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Tested-by: Philipp Hahn <pmhahn@pmhahn.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

Fixes: Change-Id: Ia374ee061195088f8c777940baa75cedbe897f4e
       ("UPSTREAM: unix: avoid use-after-free in ep_remove_wait_queue")
Change-Id: I4ebef6a390df3487903b166b837e34c653e01cb2
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>