mm: zswap: remove zswap_header

mm: zswap: remove zswap_header

Previously, zswap_header served the purpose of storing the swpentry within
zpool pages.  This allowed zpool implementations to pass relevant
information to the writeback function.  However, with the current
implementation, writeback is directly handled within zswap.  Consequently,
there is no longer a necessity for zswap_header, as the swp_entry_t can be
stored directly in zswap_entry.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Domenico Cerasuolo <[email protected]>
Tested-by: Yosry Ahmed <[email protected]>
Suggested-by: Yosry Ahmed <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>