[NETLINK]: Remove dead code in af_netlink.c

Remove the variable nlk & call to nlk_sk as it does not have any side effect.

Signed-off-by: Jayachandran C. <c.jayachandran at gmail.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>