Tracking socket address family for `ioctl()`?

[eqvinox]

Related to #3615 - there's a bunch of socket family specific ioctls:

$ pcregrep -r 'SIOCPROTOPRIVATE\s*\+\s*1\W' /usr/include/linux* 
/usr/include/linux/ax25.h:#define SIOCAX25ADDUID		(SIOCPROTOPRIVATE+1)
/usr/include/linux/rose.h:#define	SIOCRSSCAUSE		(SIOCPROTOPRIVATE+1)
/usr/include/linux/kcm.h:#define SIOCKCMUNATTACH	(SIOCPROTOPRIVATE + 1)
/usr/include/linux/mroute6.h:#define SIOCGETSGCNT_IN6	(SIOCPROTOPRIVATE+1)
/usr/include/linux/mctp.h:#define SIOCMCTPDROPTAG		(SIOCPROTOPRIVATE + 1)
/usr/include/linux/rds.h:#define SIOCRDSGETTOS		(SIOCPROTOPRIVATE + 1)
/usr/include/linux/tipc.h:#define SIOCGETNODEID          (SIOCPROTOPRIVATE + 1)
/usr/include/linux/x25.h:#define	SIOCX25SSUBSCRIP	(SIOCPROTOPRIVATE + 1)
/usr/include/linux/mroute.h:#define SIOCGETSGCNT	(SIOCPROTOPRIVATE+1)

Is there some existing way to track the address family for a fd when it is a socket? (I don't think I can implement that myself)