mm/zswap: cleanup zswap_writeback_entry()

mm/zswap: cleanup zswap_writeback_entry()

Also after the common decompress part goes to __zswap_load(), we can
cleanup the zswap_writeback_entry() a little.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Chengming Zhou <[email protected]>
Reviewed-by: Yosry Ahmed <[email protected]>
Reviewed-by: Nhat Pham <[email protected]>
Acked-by: Chris Li <[email protected]> (Google)
Cc: Barry Song <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>