FROMGIT: erofs: fix ztailpacking for subpage compressed blocks

`pageofs_in` should be the compressed data offset of the page rather
than of the block.

Acked-by: Chao Yu <chao@kernel.org>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20231214161337.753049-1-hsiangkao@linux.alibaba.com

Bug: 318378021
Change-Id: I0997a69b22b0f42c327c810359f55f5fa6a76275
(cherry picked from commit e5aba911
 https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs.git

 dev)
Signed-off-by: Sandeep Dhavale <dhavale@google.com>