Use an accessor function to access ifr_data.

This fixes 32-bit compat (no ioctl command defintions are required
as struct ifreq is the same size).  This is believed to be sufficent to
fully support ifconfig on 32-bit systems.

Reviewed by:	kib
Obtained from:	CheriBSD
MFC after:	1 week
Relnotes:	yes
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14900

newlib/libc/sys/rtems/include/net/if.h

@@ -412,7 +412,9 @@ struct	ifreq {
 #define	ifr_mtu		ifr_ifru.ifru_mtu	/* mtu */
 #define ifr_phys	ifr_ifru.ifru_phys	/* physical wire */
 #define ifr_media	ifr_ifru.ifru_media	/* physical media */
+#ifndef _KERNEL
 #define	ifr_data	ifr_ifru.ifru_data	/* for use by interface */
+#endif
 #define	ifr_reqcap	ifr_ifru.ifru_cap[0]	/* requested capabilities */
 #define	ifr_curcap	ifr_ifru.ifru_cap[1]	/* current capabilities */
 #define	ifr_index	ifr_ifru.ifru_index	/* interface index */