LU-10391 lnet: fix build issue when IPv6 is disabled.

LU-10391 lnet: fix build issue when IPv6 is disabled.

struct inet6_dev and struct inet6_ifaddr are not defined if IPv6
is not configured for the Linux kernel.

Test-Parameters: trivial
Fixes: 781499eee64 ("LU-10391 lnet: support IPv6 in lnet_inet_enumerate()")
Change-Id: I8b16ad7bea1394c4560130190023590213ff2ded
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48990
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>