hlist_head/hlist_node is the preferred data structure
for hash tables. Not only does it make the 'head' smaller,
but is also provides hlist_unhashed() which can be used to
check if an object is in the list.  This means that
we don't need h_in any more.

Change-Id: I18e2799a6e719b96ed47747375e4e20675d9b7cc
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35862


Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>