esp: drop unneeded assignment in esp4_gro_receive()

Making '!=' operation with 0 directly after calling
the function xfrm_parse_spi() is more efficient,
assignment to err is redundant.

Eliminate the following clang_analyzer warning:
net/ipv4/esp4_offload.c:41:7: warning: Although the value stored to
'err' is used in the enclosing expression, the value is never actually
read from 'err'

No functional change, only more efficient.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/esp4_offload.c

@@ -33,12 +33,11 @@ static struct sk_buff *esp4_gro_receive(struct list_head *head,
 	struct xfrm_state *x;
 	__be32 seq;
 	__be32 spi;
-	int err;
 
 	if (!pskb_pull(skb, offset))
 		return NULL;
 
-	if ((err = xfrm_parse_spi(skb, IPPROTO_ESP, &spi, &seq)) != 0)
+	if (xfrm_parse_spi(skb, IPPROTO_ESP, &spi, &seq) != 0)
 		goto out;
 
 	xo = xfrm_offload(skb);