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

net/netlink/genetlink.c

@@ -136,7 +136,7 @@ int genl_register_mc_group(struct genl_family *family,
 {
 	int id;
 	unsigned long *new_groups;
-	int err;
+	int err = 0;
 
 	BUG_ON(grp->name[0] == '\0');