tcp: inline tcp_filter()

This helper is already (auto)inlined from IPv4 TCP stack.

Make it an inline function to benefit IPv6 as well.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/2 grow/shrink: 1/0 up/down: 30/-49 (-19)
Function                                     old     new   delta
tcp_v6_rcv                                  3448    3478     +30
__pfx_tcp_filter                              16       -     -16
tcp_filter                                    33       -     -33
Total: Before=24891904, After=24891885, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260205164329.3401481-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/net/tcp.h

@@ -1633,8 +1633,14 @@ static inline bool tcp_checksum_complete(struct sk_buff *skb)
 bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
 		     enum skb_drop_reason *reason);
 
+static inline int tcp_filter(struct sock *sk, struct sk_buff *skb,
+			     enum skb_drop_reason *reason)
+{
+	const struct tcphdr *th = (const struct tcphdr *)skb->data;
+
+	return sk_filter_trim_cap(sk, skb, __tcp_hdrlen(th), reason);
+}
 
-int tcp_filter(struct sock *sk, struct sk_buff *skb, enum skb_drop_reason *reason);
 void tcp_set_state(struct sock *sk, int state);
 void tcp_done(struct sock *sk);
 int tcp_abort(struct sock *sk, int err);

net/ipv4/tcp_ipv4.c

@@ -2109,14 +2109,6 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
 }
 EXPORT_IPV6_MOD(tcp_add_backlog);
 
-int tcp_filter(struct sock *sk, struct sk_buff *skb, enum skb_drop_reason *reason)
-{
-	struct tcphdr *th = (struct tcphdr *)skb->data;
-
-	return sk_filter_trim_cap(sk, skb, th->doff * 4, reason);
-}
-EXPORT_IPV6_MOD(tcp_filter);
-
 static void tcp_v4_restore_cb(struct sk_buff *skb)
 {
 	memmove(IPCB(skb), &TCP_SKB_CB(skb)->header.h4,