net: add sk_stream_is_writeable() helper

Several call sites use the hardcoded following condition :

sk_stream_wspace(sk) >= sk_stream_min_wspace(sk)

Lets use a helper because TCP_NOTSENT_LOWAT support will change this
condition for TCP sockets.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 305374d..0da6785 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -442,7 +442,7 @@
 {
 	struct socket *sock = sk->sk_socket;
 
-	if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk) && sock)
+	if (sk_stream_is_writeable(sk) && sock)
 		clear_bit(SOCK_NOSPACE, &sock->flags);
 	svc_write_space(sk);
 }