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>

net/netfilter/nf_nat_helper.c

@@ -37,7 +37,7 @@ static void mangle_contents(struct sk_buff *skb,
 {
 	unsigned char *data;
 
-	BUG_ON(skb_is_nonlinear(skb));
+	SKB_LINEAR_ASSERT(skb);
 	data = skb_network_header(skb) + dataoff;
 
 	/* move post-replacement */
@@ -110,8 +110,6 @@ bool __nf_nat_mangle_tcp_packet(struct sk_buff *skb,
 	    !enlarge_skb(skb, rep_len - match_len))
 		return false;
 
-	SKB_LINEAR_ASSERT(skb);
-
 	tcph = (void *)skb->data + protoff;
 
 	oldlen = skb->len - protoff;