xfrm: free skb if nlsk pointer is NULL

xfrm: free skb if nlsk pointer is NULL

[ Upstream commit 86126b77dcd551ce223e7293bb55854e3df05646 ]

nlmsg_multicast() always frees the skb, so in case we cannot call
it we must do that ourselves.

Fixes: 21ee543edc0dea ("xfrm: fix race between netns cleanup and state expire notification")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>