support setting read flag through ioctl

[jiefenghuang]

What would you like to be added:
As an explicit option, it allows users to pass hints about read access patterns—akin to posix_fadvise—as supplementary preconditions for readahead strategy, enabling the readahead mechanism to better adapt to different read modes.

Why is this needed:
An explicit option allows the readahead strategy to become more precise.