tcp: Slightly optimize tcp_sendmsg

Slightly optimize tcp_sendmsg since NETIF_F_SG is used many
times iteratively in the loop. The only other modification is
to change:
			} else if (i == MAX_SKB_FRAGS ||
				   (!i &&
				   !(sk->sk_route_caps & NETIF_F_SG))) {
	to:
			} else if (i == MAX_SKB_FRAGS || !sg) {

The reason why this change is correct: this code (other than
the MAX_SKB_FRAGS case) executes only due to the else part
of: "if (skb_tailroom(skb) > 0) {" - i.e. there was no space
in the skb to put the data inline. Hence SG is false is a
sufficient condition, and there is no way a fragment can be
added to the skb.

Changelog:
	- Added the above explanation for the change
Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -876,12 +876,12 @@ ssize_t tcp_sendpage(struct socket *sock, struct page *page, int offset,
 #define TCP_PAGE(sk)	(sk->sk_sndmsg_page)
 #define TCP_OFF(sk)	(sk->sk_sndmsg_off)
 
-static inline int select_size(struct sock *sk)
+static inline int select_size(struct sock *sk, int sg)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	int tmp = tp->mss_cache;
 
-	if (sk->sk_route_caps & NETIF_F_SG) {
+	if (sg) {
 		if (sk_can_gso(sk))
 			tmp = 0;
 		else {
@@ -905,7 +905,7 @@ int tcp_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
 	struct sk_buff *skb;
 	int iovlen, flags;
 	int mss_now, size_goal;
-	int err, copied;
+	int sg, err, copied;
 	long timeo;
 
 	lock_sock(sk);
@@ -933,6 +933,8 @@ int tcp_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
 	if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))
 		goto out_err;
 
+	sg = sk->sk_route_caps & NETIF_F_SG;
+
 	while (--iovlen >= 0) {
 		int seglen = iov->iov_len;
 		unsigned char __user *from = iov->iov_base;
@@ -958,7 +960,8 @@ int tcp_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
 				if (!sk_stream_memory_free(sk))
 					goto wait_for_sndbuf;
 
-				skb = sk_stream_alloc_skb(sk, select_size(sk),
+				skb = sk_stream_alloc_skb(sk,
+							  select_size(sk, sg),
 							  sk->sk_allocation);
 				if (!skb)
 					goto wait_for_memory;
@@ -996,9 +999,7 @@ int tcp_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
 					/* We can extend the last page
 					 * fragment. */
 					merge = 1;
-				} else if (i == MAX_SKB_FRAGS ||
-					   (!i &&
-					   !(sk->sk_route_caps & NETIF_F_SG))) {
+				} else if (i == MAX_SKB_FRAGS || !sg) {
 					/* Need to add new fragment and cannot
 					 * do this because interface is non-SG,
 					 * or because all the page slots are