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>

include/uapi/linux/genetlink.h

@@ -26,7 +26,6 @@ struct genlmsghdr {
 /*
  * List of reserved static generic netlink identifiers:
  */
-#define GENL_ID_GENERATE	0
 #define GENL_ID_CTRL		NLMSG_MIN_TYPE
 #define GENL_ID_VFS_DQUOT	(NLMSG_MIN_TYPE + 1)
 #define GENL_ID_PMCRAID		(NLMSG_MIN_TYPE + 2)