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/ceph/messenger.c b/net/ceph/messenger.c
index eb0a46a..3be308e 100644
--- a/net/ceph/messenger.c
+++ b/net/ceph/messenger.c
@@ -409,7 +409,7 @@
 	 * and net/core/stream.c:sk_stream_write_space().
 	 */
 	if (con_flag_test(con, CON_FLAG_WRITE_PENDING)) {
-		if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk)) {
+		if (sk_stream_is_writeable(sk)) {
 			dout("%s %p queueing write work\n", __func__, con);
 			clear_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 			queue_con(con);