mm/z3fold: add kerneldoc fields for z3fold_pool

make W=1 generates the following warning for z3fold_pool

  mm/z3fold.c:171: warning: Function parameter or member 'zpool' not described in 'z3fold_pool'
  mm/z3fold.c:171: warning: Function parameter or member 'zpool_ops' not described in 'z3fold_pool'

Commit 9a001fc19c ("z3fold: the 3-fold allocator for compressed pages")
simply did not document the fields at the time.  Add rudimentary
documentation.

Link: https://lkml.kernel.org/r/20210520084809.8576-11-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -144,6 +144,8 @@ struct z3fold_header {
  * @c_handle:	cache for z3fold_buddy_slots allocation
  * @ops:	pointer to a structure of user defined operations specified at
  *		pool creation time.
+ * @zpool:	zpool driver
+ * @zpool_ops:	zpool operations structure with an evict callback
  * @compact_wq:	workqueue for page layout background optimization
  * @release_wq:	workqueue for safe page release
  * @work:	work_struct for safe page release