genetlink: no longer support using static family IDs

Static family IDs have never really been used, the only
use case was the workaround I introduced for those users
that assumed their family ID was also their multicast
group ID.

Additionally, because static family IDs would never be
reserved by the generic netlink code, using a relatively
low ID would only work for built-in families that can be
registered immediately after generic netlink is started,
which is basically only the control family (apart from
the workaround code, which I also had to add code for so
it would reserve those IDs)

Thus, anything other than GENL_ID_GENERATE is flawed and
luckily not used except in the cases I mentioned. Move
those workarounds into a few lines of code, and then get
rid of GENL_ID_GENERATE entirely, making it more robust.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

fs/quota/netlink.c

@@ -13,13 +13,6 @@ static const struct genl_multicast_group quota_mcgrps[] = {
 
 /* Netlink family structure for quota */
 static struct genl_family quota_genl_family = {
-	/*
-	 * Needed due to multicast group ID abuse - old code assumed
-	 * the family ID was also a valid multicast group ID (which
-	 * isn't true) and userspace might thus rely on it. Assign a
-	 * static ID for this group to make dealing with that easier.
-	 */
-	.id = GENL_ID_VFS_DQUOT,
 	.hdrsize = 0,
 	.name = "VFS_DQUOT",
 	.version = 1,