[INET]: Introduce inet_sk_rebuild_header

From tcp_v4_rebuild_header, that already was pretty generic, I only
needed to use sk->sk_protocol instead of the hardcoded IPPROTO_TCP and
establish the requirement that INET transport layer protocols that
want to use this function map TCP_SYN_SENT to its equivalent state.

Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/ip.h b/include/linux/ip.h
index 31e7ced..33e8a19 100644
--- a/include/linux/ip.h
+++ b/include/linux/ip.h
@@ -196,6 +196,8 @@
 #endif
 #endif
 
+extern int inet_sk_rebuild_header(struct sock *sk);
+
 struct iphdr {
 #if defined(__LITTLE_ENDIAN_BITFIELD)
 	__u8	ihl:4,