UPSTREAM: zram: refactor flags and writeback stuff

Rename some variables and restructure some code for better readability in
writeback and zs_free_page.

Link: http://lkml.kernel.org/r/20181127055429.251614-4-minchan@kernel.org


Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 7e5292831b346bacb05558ca385cae366187314c)

Bug: 117683045
Change-Id: I4ff293133bbd2a6d43a4860220904c08bb381bd0
Signed-off-by: Srinivas Paladugu <srnvs@google.com>