mm, swap: add swap_cluster_list

mm, swap: add swap_cluster_list

This is a code clean up patch without functionality changes.  The
swap_cluster_list data structure and its operations are introduced to
provide some better encapsulation for the free cluster and discard cluster
list operations.  This avoid some code duplication, improved the code
readability, and reduced the total line number.

Link: http://lkml.kernel.org/r/1472067356-16004-1-git-send-email-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shaohua Li <shli@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>