netfilter: nat: remove duplicate skb_is_nonlinear() in __nf_nat_mangle_tcp_packet()

__nf_nat_mangle_tcp_packet() and nf_nat_mangle_udp_packet() call
mangle_contents(). and __nf_nat_mangle_tcp_packet()
and mangle_contents() call skb_is_nonlinear(). so that
skb_is_nonlinear() in __nf_nat_mangle_tcp_packet() is unnecessary.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>