LU-14095 gss: use hlist_unhashed() instead of ->next

LU-14095 gss: use hlist_unhashed() instead of ->next

In cache_detail list-mutation primitives, verifying the status of an
entry must be done using hlist_unhashed(), in case 'struct cache_head'
has a 'cache_list' field.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1410eca9a647b74127cf40b8f3d6b68d055f773a
Reviewed-on: https://review.whamcloud.com/40514
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>