update iprule.sh.

fixed bug: some websites can not be accessed, for computers behind
NAT.

root cause: Large TCP packets lost, due to wrong MSS.

Solution:  add a rule in mangle table
iptables -t mangle -I FORWARD -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu