tcp: tcp_init_buffer_space can be static

As of commit 98fa6271
("tcp: refactor setting the initial congestion window") this is called
only from tcp_input.c, so it can be static.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>