gro: fix napi_gro_frags() Fast GRO breakage due to IP alignment check

Commit 38ec4944 ("gro: ensure frag0 meets IP header alignment")
did the right thing, but missed the fact that napi_gro_frags() logics
calls for skb_gro_reset_offset() *before* pulling Ethernet header
to the skb linear space.
That said, the introduced check for frag0 address being aligned to 4
always fails for it as Ethernet header is obviously 14 bytes long,
and in case with NET_IP_ALIGN its start is not aligned to 4.

Fix this by adding @nhoff argument to skb_gro_reset_offset() which
tells if an IP header is placed right at the start of frag0 or not.
This restores Fast GRO for napi_gro_frags() that became very slow
after the mentioned commit, and preserves the introduced check to
avoid silent unaligned accesses.

From v1 [0]:
 - inline tiny skb_gro_reset_offset() to let the code be optimized
   more efficively (esp. for the !NET_IP_ALIGN case) (Eric);
 - pull in Reviewed-by from Eric.

[0] https://lore.kernel.org/netdev/20210418114200.5839-1-alobakin@pm.me

Fixes: 38ec4944 ("gro: ensure frag0 meets IP header alignment")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5914,7 +5914,7 @@ static struct list_head *gro_list_prepare(struct napi_struct *napi,
 	return head;
 }
 
-static void skb_gro_reset_offset(struct sk_buff *skb)
+static inline void skb_gro_reset_offset(struct sk_buff *skb, u32 nhoff)
 {
 	const struct skb_shared_info *pinfo = skb_shinfo(skb);
 	const skb_frag_t *frag0 = &pinfo->frags[0];
@@ -5925,7 +5925,7 @@ static void skb_gro_reset_offset(struct sk_buff *skb)
 
 	if (!skb_headlen(skb) && pinfo->nr_frags &&
 	    !PageHighMem(skb_frag_page(frag0)) &&
-	    (!NET_IP_ALIGN || !(skb_frag_off(frag0) & 3))) {
+	    (!NET_IP_ALIGN || !((skb_frag_off(frag0) + nhoff) & 3))) {
 		NAPI_GRO_CB(skb)->frag0 = skb_frag_address(frag0);
 		NAPI_GRO_CB(skb)->frag0_len = min_t(unsigned int,
 						    skb_frag_size(frag0),
@@ -6143,7 +6143,7 @@ gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
 	skb_mark_napi_id(skb, napi);
 	trace_napi_gro_receive_entry(skb);
 
-	skb_gro_reset_offset(skb);
+	skb_gro_reset_offset(skb, 0);
 
 	ret = napi_skb_finish(napi, skb, dev_gro_receive(napi, skb));
 	trace_napi_gro_receive_exit(ret);
@@ -6232,7 +6232,7 @@ static struct sk_buff *napi_frags_skb(struct napi_struct *napi)
 	napi->skb = NULL;
 
 	skb_reset_mac_header(skb);
-	skb_gro_reset_offset(skb);
+	skb_gro_reset_offset(skb, hlen);
 
 	if (unlikely(skb_gro_header_hard(skb, hlen))) {
 		eth = skb_gro_header_slow(skb, hlen, 0);