Fix access to ifru_buffer on freebsd32.

Make all kernel accesses to ifru_buffer go via access functions
which take the process ABI into account and use an appropriate union
to access members in the correct place in struct ifreq.

Reviewed by:	kib
Obtained from:	CheriBSD
MFC after:	1 week
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14846

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

@@ -402,7 +402,9 @@ struct	ifreq {
 #define	ifr_addr	ifr_ifru.ifru_addr	/* address */
 #define	ifr_dstaddr	ifr_ifru.ifru_dstaddr	/* other end of p-to-p link */
 #define	ifr_broadaddr	ifr_ifru.ifru_broadaddr	/* broadcast address */
+#ifndef _KERNEL
 #define	ifr_buffer	ifr_ifru.ifru_buffer	/* user supplied buffer with its length */
+#endif
 #define	ifr_flags	ifr_ifru.ifru_flags[0]	/* flags (low 16 bits) */
 #define	ifr_flagshigh	ifr_ifru.ifru_flags[1]	/* flags (high 16 bits) */
 #define	ifr_jid		ifr_ifru.ifru_jid	/* jail/vnet */