IPv6 support for NFS server export caches

IPv6 support for NFS server export caches

This adds IPv6 support to the interfaces that are used to express nfsd
exports.  All addressed are stored internally as IPv6; backwards
compatibility is maintained using mapped addresses.

Thanks to Bruce Fields, Brian Haley, Neil Brown and Hideaki Joshifuji
for comments

Signed-off-by: Aurelien Charbon <aurelien.charbon@bull.net>
Cc: Neil Brown <neilb@suse.de>
Cc: Brian Haley <brian.haley@hp.com>
Cc:  YOSHIFUJI Hideaki / 吉藤英明 <yoshfuji@linux-ipv6.org>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>