mm/zswap: refactor the get/put routines

mm/zswap: refactor the get/put routines

The refcount routine was not fit the kernel get/put semantic exactly,
There were too many judgement statements on refcount and it could be
minus.

This patch does the following:

- move refcount judgement to zswap_entry_put() to hide resource free function.

- add a new function zswap_entry_find_get(), so that callers can use easily
in the following pattern:

   zswap_entry_find_get
   .../* do something */
   zswap_entry_put

- to eliminate compile error, move some functions declaration

This patch is based on Minchan Kim <minchan@kernel.org> 's idea and suggestion.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Cc: Seth Jennings <sjennings@variantweb.net>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>