genetlink: fix error return code in genl_register_family()

Fix to return a negative error code from the idr_alloc() error handling
case instead of 0, as done elsewhere in this function.

Also fix the return value check of idr_alloc() since idr_alloc return
negative errors on failure, not zero.

Fixes: 2ae0f17df1 ("genetlink: use idr to track families")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -362,8 +362,10 @@ int genl_register_family(struct genl_family *family)
 
 	family->id = idr_alloc(&genl_fam_idr, family,
 			       start, end + 1, GFP_KERNEL);
-	if (!family->id)
+	if (family->id < 0) {
+		err = family->id;
 		goto errout_locked;
+	}
 
 	err = genl_validate_assign_mc_groups(family);
 	if (err)