genetlink: limit the use of validation workarounds to old ops

During review of previous change another thing came up - we should
limit the use of validation workarounds to old commands.
Don't list the workarounds one by one, as we're rejecting all existing
ones. We can deal with the masking in the unlikely event that new flag
is added.

Link: https://lore.kernel.org/all/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/netlink/genetlink.c

···
 		genl_get_cmd_by_index(i, family, &op);
 		if (op.dumpit == NULL && op.doit == NULL)
 			return -EINVAL;
+		if (WARN_ON(op.cmd >= family->resv_start_op && op.validate))
+			return -EINVAL;
 		for (j = i + 1; j < genl_get_cmd_cnt(family); j++) {
 			struct genl_ops op2;