gtp: put back reference to netns when not required anymore

This patch fixes a netns leak.

Fixes: 93edb8c7 ("gtp: reload GTPv1 header after pskb_may_pull()")
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>