mm/zbud: add kerneldoc fields for zbud_pool

make W=1 generates the following warning for zbud_pool

  mm/zbud.c:105: warning: Function parameter or member 'zpool' not described in 'zbud_pool'
  mm/zbud.c:105: warning: Function parameter or member 'zpool_ops' not described in 'zbud_pool'

Commit 479305fd ("zpool: remove zpool_evict()") removed the
zpool_evict helper and added the associated zpool and operations structure
in struct zbud_pool but did not add documentation for the fields.  Add
rudimentary documentation.

Link: https://lkml.kernel.org/r/20210520084809.8576-10-mgorman@techsingularity.net
Fixes: 479305fd

 ("zpool: remove zpool_evict()")
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>