erofs: get rid of erofs_prepare_dio() helper

Fold in erofs_prepare_dio() in order to simplify the code.

Reviewed-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220720082229.12172-1-hsiangkao@linux.alibaba.com