netfilter: xt_TCPMSS: correct return value in tcpmss_mangle_packet

In commit b396966c4 (netfilter: xt_TCPMSS: Fix missing fragmentation handling),
I attempted to add safe fragment handling to xt_TCPMSS.  However, Andy Padavan
of Project N56U correctly points out that returning XT_CONTINUE in this
function does not work.  The callers (tcpmss_tg[46]) expect to receive a value
of 0 in order to return XT_CONTINUE.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>