net: make tcp_cleanup_rbuf private

net_dma was the only external user so this can become local to tcp.c
again.

Cc: James Morris <jmorris@namei.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

include/net/tcp.h

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 2c2f24ff..7ffbcbf 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -371,7 +371,6 @@
 void tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
 			 const struct tcphdr *th, unsigned int len);
 void tcp_rcv_space_adjust(struct sock *sk);
-void tcp_cleanup_rbuf(struct sock *sk, int copied);
 int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp);
 void tcp_twsk_destructor(struct sock *sk);
 ssize_t tcp_splice_read(struct socket *sk, loff_t *ppos,

net/ipv4/tcp.c

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 28595a3..4874a0e 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1377,7 +1377,7 @@
  * calculation of whether or not we must ACK for the sake of
  * a window update.
  */
-void tcp_cleanup_rbuf(struct sock *sk, int copied)
+static void tcp_cleanup_rbuf(struct sock *sk, int copied)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	bool time_to_ack = false;