genetlink: Optimize genl_register_family()

genl_register_family() doesn't need to call genl_family_find_byid
when GENL_ID_GENERATE is passed during register.

Patch on net-next-2.6, compile and reboot testing only.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>