[TCP_IPV4]: Use kmemdup where appropriate

Also use a variable to avoid the longish tp->md5sig_info-> use
in tcp_v4_md5_do_add.

Code diff stats:

[acme@newtoy net-2.6.20]$ codiff /tmp/tcp_ipv4.o.before /tmp/tcp_ipv4.o.after
/pub/scm/linux/kernel/git/acme/net-2.6.20/net/ipv4/tcp_ipv4.c:
  tcp_v4_md5_do_add     |  -62
  tcp_v4_syn_recv_sock  |  -32
  tcp_v4_parse_md5_keys |  -86
 3 functions changed, 180 bytes removed
[acme@newtoy net-2.6.20]$

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>