netfilter: xt_TCPMSS: Get mtu only if clamp-mss-to-pmtu is specified

This patch refactors the code to skip tcpmss_reverse_mtu if no
clamp-mss-to-pmtu is specified.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>