zram: add a huge_idle writeback mode

zram: add a huge_idle writeback mode

Today it's only possible to write back as a page, idle, or huge.  A user
might want to writeback pages which are huge and idle first as these idle
pages do not require decompression and make a good first pass for
writeback.

Idle writeback specifically has the advantage that a refault is unlikely
given that the page has been swapped for some amount of time without being
refaulted.

Huge writeback has the advantage that you're guaranteed to get the maximum
benefit from a single page writeback, that is, you're reclaiming one full
page of memory.  Pages which are compressed in zram being written back
result in some benefit which is always less than a page size because of
the fact that it was compressed.

The primary use of this is for minimizing refaults in situations where the
device has to be sensitive to storage endurance.  On ChromeOS we have
devices with slow eMMC and repeated writes and refaults can negatively
affect performance and endurance.

Link: https://lkml.kernel.org/r/20220322215821.1196994-1-bgeffon@google.com
Signed-off-by: Brian Geffon <bgeffon@google.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>