erofs: record `pclustersize` in bytes instead of pages

Currently, compressed sizes are recorded in pages using `pclusterpages`,
However, for tailpacking pclusters, `tailpacking_size` is used instead.

This approach doesn't work when dealing with sub-page blocks. To address
this, let's switch them to the unified `pclustersize` in bytes.

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20231206091057.87027-3-hsiangkao@linux.alibaba.com