ipv6: exthdrs: Reload hdr only when needed in ipv6_srh_rcv().

We need not reload hdr in ipv6_srh_rcv() unless we call
pskb_expand_head().
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/exthdrs.c

@@ -440,9 +440,9 @@ static int ipv6_srh_rcv(struct sk_buff *skb)
 			kfree_skb(skb);
 			return -1;
 		}
-	}
 
-	hdr = (struct ipv6_sr_hdr *)skb_transport_header(skb);
+		hdr = (struct ipv6_sr_hdr *)skb_transport_header(skb);
+	}
 
 	hdr->segments_left--;
 	addr = hdr->segments + hdr->segments_left;