zram: fix the issue that the write - back limits might overflow

zram: fix the issue that the write - back limits might overflow

When the page size exceeds 4KB, if bd_wb_limit is set to a value that is
not aligned with the page size, it will cause a numerical wrap-around
issue for bd_wb_limit.  For example, when the page size is set to 16KB and
bd_wb_limit is set to 3, after one write-back operation, the value of
bd_wb_limit will become -1.  More seriously, since bd_wb_limit is an
unsigned number, its value may become as large as 2^64 - 1.

The core reason for this problem is that the unit of bd_wb_limit is 4KB.
For example, when a write-back occurs on a system with a page size of
16KB, 4 needs to be subtracted from bd_wb_limit.  This operation takes
place in the zram_account_writeback_submit function.

This patch fixes the issue by limiting bd_wb_limit to be an integer
multiple of PAGE_SIZE / 4096.

Link: https://lkml.kernel.org/r/tencent_5936CFE72BAB2BA76887BB69DCC1B5E67C05@qq.com
Fixes: 1d69a3f8ae77 ("zram: idle writeback fixes and cleanup")
Signed-off-by: Yuwen Chen <ywen.chen@foxmail.com>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>