To prepare for the upcoming ztailpacking feature, introduce
z_erofs_fixup_insize() and pageofs_in to wrap up the process
to get the exact compressed size via zero padding.

Link: https://lore.kernel.org/r/20211228054604.114518-3-hsiangkao@linux.alibaba.comReviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>