erofs: move preparation logic into z_erofs_pcluster_begin()

Some preparation logic should be part of z_erofs_pcluster_begin()
instead of z_erofs_do_read_page().  Let's move now.

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/20230817082813.81180-3-hsiangkao@linux.alibaba.com