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/include/net/sock.h b/include/net/sock.h
index e0473f6..d0b5fde 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1088,6 +1088,10 @@
 }
 #endif
 
+static inline bool sk_stream_is_writeable(const struct sock *sk)
+{
+	return sk_stream_wspace(sk) >= sk_stream_min_wspace(sk);
+}
 
 static inline bool sk_has_memory_pressure(const struct sock *sk)
 {