[DECNET]: Use kmemdup

Code diff stats:

[acme@newtoy net-2.6.20]$ codiff /tmp/decnet.ko.before /tmp/decnet.ko.after
/pub/scm/linux/kernel/git/acme/net-2.6.20/net/decnet/dn_dev.c:
  dn_dev_sysctl_register |  -51
 1 function changed, 51 bytes removed
[acme@newtoy net-2.6.20]$

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