lsm: clean up redundant NULL pointer check

The implements of {ip,tcp,udp,dccp,sctp,ipv6}_hdr(skb) guarantee that
they will never return NULL, and elsewhere users don't do the check
as well, so remove the check here.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
[PM: subject line tweaks]
Signed-off-by: Paul Moore <paul@paul-moore.com>