tcp: avoid indirect call in tcp_new_space()

For tcp sockets, sk->sk_write_space is most probably sk_stream_write_space().

Other sk->sk_write_space() calls in TCP are slow path and do not deserve
any change.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>