tcp: make tcp_build_frag() static

After the previous patch the mentioned helper is
used only inside its compilation unit: let's make
it static.

RFC -> v1:
 - preserve the tcp_build_frag() helper (Eric)
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -330,8 +330,6 @@ int tcp_sendpage(struct sock *sk, struct page *page, int offset, size_t size,
 		 int flags);
 int tcp_sendpage_locked(struct sock *sk, struct page *page, int offset,
 			size_t size, int flags);
-struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
-			       struct page *page, int offset, size_t *size);
 ssize_t do_tcp_sendpages(struct sock *sk, struct page *page, int offset,
 		 size_t size, int flags);
 int tcp_send_mss(struct sock *sk, int *size_goal, int flags);

net/ipv4/tcp.c

@@ -963,8 +963,8 @@ void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb)
 	}
 }
 
-struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
-			       struct page *page, int offset, size_t *size)
+static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
+				      struct page *page, int offset, size_t *size)
 {
 	struct sk_buff *skb = tcp_write_queue_tail(sk);
 	struct tcp_sock *tp = tcp_sk(sk);