netlink: silence compiler warning

  CC      net/netlink/genetlink.o
net/netlink/genetlink.c: In function ‘genl_register_mc_group’:
net/netlink/genetlink.c:139: warning: ‘err’ may be used uninitialized in this function

From following the code 'err' is initialized, but set it to zero to
silence the warning.

Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>