IPVS netns exit causes crash in conntrack

commit 8f4e0a18 upstream.

Quote from Patric Mc Hardy
"This looks like nfnetlink.c excited and destroyed the nfnl socket, but
ip_vs was still holding a reference to a conntrack. When the conntrack
got destroyed it created a ctnetlink event, causing an oops in
netlink_has_listeners when trying to use the destroyed nfnetlink
socket."

If nf_conntrack_netlink is loaded before ip_vs this is not a problem.

This patch simply avoids calling ip_vs_conn_drop_conntrack()
when netns is dying as suggested by Julian.

Signed-off-by: Hans Schillstrom <hans.schillstrom@ericsson.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>