netfilter: xt_TCPMSS: Fix IPv6 default MSS too

As a followup to commit 409b545a ("netfilter: xt_TCPMSS: Fix violation
of RFC879 in absence of MSS option"), John Heffner points out that IPv6
has a higher MTU than IPv4, and thus a higher minimum MSS. Update TCPMSS
target to account for this, and update RFC comment.

While at it, point to more recent reference RFC1122 instead of RFC879.

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