ipvs: Fix panic due to non-linear skb

ipvs: Fix panic due to non-linear skb

commit f44a5f45f544561302e855e7bd104e5f506ec01b upstream.

Receiving a ICMP response to an IPIP packet in a non-linear skb could
cause a kernel panic in __skb_pull.

The problem was introduced in
commit f2edb9f7706dcb2c0d9a362b2ba849efe3a97f5e ("ipvs: implement
passive PMTUD for IPIP packets").

Signed-off-by: Peter Christensen <pch@ordbogen.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>