tcp: use TCP_BASE_MSS to set basic mss value

TCP_BASE_MSS is defined, but not used.
commit 5d424d5a introduce this macro, so use
it to initial sysctl_tcp_base_mss.

commit 5d424d5a
Author: John Heffner <jheffner@psc.edu>
Date:   Mon Mar 20 17:53:41 2006 -0800

    [TCP]: MTU probing

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>