mm: zswap: use object read/write APIs instead of object mapping APIs

Use the new object read/write APIs instead of mapping APIs.

On compress side, zpool_obj_write() is more concise and provides exactly
what zswap needs to write the compressed object to the zpool, instead of
map->copy->unmap.

On the decompress side, zpool_obj_read_begin() is sleepable, which
allows avoiding the memcpy() for zsmalloc and slightly simplifying the
code by:
- Avoiding checking if the zpool driver is sleepable, reducing special
  cases and shrinking the huge comment.
- Having a single zpool_obj_read_end() call rather than multiple
  conditional zpool_unmap_handle() calls.

The !virt_addr_valid() case can be removed in the future if the crypto API
supports kmap addresses or by using kmap_to_page(), completely eliminating
the memcpy() path in zswap_decompress().  This a step toward that.  In
that spirit, opportunistically make the comment more specific about the
kmap case instead of generic non-linear addresses.  This is the only case
that needs to be handled in practice, and the generic comment makes it
seem like a bigger problem that it actually is.

Link: https://lkml.kernel.org/r/20250305061134.4105762-3-yosry.ahmed@linux.dev


Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>