NETFILTER: xt_TCPMSS: remove network triggerable WARN_ON

[NETFILTER]: xt_TCPMSS: remove network triggerable WARN_ON

[ Upstream commit: 9dc0564e ]

ipv6_skip_exthdr() returns -1 for invalid packets. don't WARN_ON
that.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>