genetlink: Fix uninitialized variable in genl_validate_assign_mc_groups()

net/netlink/genetlink.c: In function ‘genl_validate_assign_mc_groups’:
net/netlink/genetlink.c:217: warning: ‘err’ may be used uninitialized in this
function

Commit 2a94fe48f32ccf7321450a2cc07f2b724a444e5b ("genetlink: make multicast
groups const, prevent abuse") split genl_register_mc_group() in multiple
functions, but dropped the initialization of err.

Initialize err to zero to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
index 4518a57..803206e 100644
--- a/net/netlink/genetlink.c
+++ b/net/netlink/genetlink.c
@@ -214,7 +214,7 @@
 {
 	int first_id;
 	int n_groups = family->n_mcgrps;
-	int err, i;
+	int err = 0, i;
 	bool groups_allocated = false;
 
 	if (!n_groups)