staging: erofs: decompress asynchronously if PG_readahead page at first

For the case of nr_to_read == lookahead_size, it is better to
decompress asynchronously as well since no page will be needed immediately.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>