mm: zswap: use SG list decompression APIs from zsmalloc

mm: zswap: use SG list decompression APIs from zsmalloc

Use the new zs_obj_read_sg_*() APIs in zswap_decompress(), instead of
zs_obj_read_*() APIs returning a linear address.  The SG list is passed
directly to the crypto API, simplifying the logic and dropping the
workaround that copies highmem addresses to a buffer.  The crypto API
should internally linearize the SG list if needed.

This avoids the memcpy() in zsmalloc for objects spanning multiple pages,
although an equivalent operation will be done internally by acomp/scomp.
However, in the future compression algorithms could support handling
discontiguous SG lists, completely eliminating the copying for spanning
objects.

Zsmalloc fills an SG list up to 2 entries in size, so change the input SG
list to fit 2 entries.

Update the incompressible entries path to use memcpy_from_sglist() to copy
the data to the folio.  Opportunistically set dlen to PAGE_SIZE in the
same code path (rather that at the top of the function) to make it
clearer.

Drop the goto in zswap_compress() as the code now is not simple enough for
an if-else statement instead.  Rename 'decomp_ret' to 'ret' and reuse it
to keep the intermediate return value of crypto_acomp_decompress() to keep
line lengths manageable.

No functional change intended.

Link: https://lkml.kernel.org/r/20260121013615.2906368-1-yosry.ahmed@linux.dev
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>