tcp: fix tcp_v6_md5_do_lookup prototype

tcp_v6_md5_do_lookup() now takes a const socket, even if
CONFIG_TCP_MD5SIG is not set.

Fixes: b83e3deb

 ("tcp: md5: constify tcp_md5_do_lookup() socket argument")
From: Eric Dumazet <edumazet@google.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>

Signed-off-by: David S. Miller <davem@davemloft.net>