erofs: add REQ_RAHEAD flag to readahead requests

Let's add REQ_RAHEAD flag so it'd be easier to identify
readahead I/O requests in blktrace.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200919072730.24989-3-hsiangkao@redhat.com


Signed-off-by: Gao Xiang <hsiangkao@redhat.com>