gro: remove a sparse error

Fix following sparse error :

net/ipv4/af_inet.c:1410:59: warning: restricted __be16 degrades to
integer

added in commit db8caf3dbc77599
("gro: should aggregate frames without DF")

Reported-by: kbuild test robot <fengguang.wu@intel.com>
From: Eric Dumazet <edumazet@google.com>

Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -1407,7 +1407,7 @@ static struct sk_buff **inet_gro_receive(struct sk_buff **head,
 		NAPI_GRO_CB(p)->flush |=
 			(iph->ttl ^ iph2->ttl) |
 			(iph->tos ^ iph2->tos) |
-			((iph->frag_off ^ iph2->frag_off) & htons(IP_DF)) |
+			(__force int)((iph->frag_off ^ iph2->frag_off) & htons(IP_DF)) |
 			((u16)(ntohs(iph2->id) + NAPI_GRO_CB(p)->count) ^ id);
 
 		NAPI_GRO_CB(p)->flush |= flush;