erofs: remove the member readahead from struct z_erofs_decompress_frontend

erofs: remove the member readahead from struct z_erofs_decompress_frontend

The struct member is only used to add REQ_RAHEAD during I/O submission.
So it is cleaner to pass it as a parameter than keep it in the struct.

Also, rename function z_erofs_get_sync_decompress_policy() to
z_erofs_is_sync_decompress() for better clarity and conciseness.

Signed-off-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230524063944.1655-1-zbestahu@gmail.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>