zsmalloc: simplify read begin/end logic

zsmalloc: simplify read begin/end logic

zs_obj_read_begin() currently maps or copies the compressed object with
the prefix handle for !ZsHugePage case.  Make the logic clearer and
more efficient by moving the offset of the object in the page after the
prefix handle instead, only copying the actual object and avoiding the
need to adjust the returned address to account for the prefix.

Adjust the logic to detect spanning objects in zs_obj_read_end()
accordingly, slightly simplifying it by avoiding the need to account
for the handle in both the offset and the object size.

Link: https://lkml.kernel.org/r/20260107052145.3586917-2-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Co-developed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>