cachefiles: add restore command to recover inflight ondemand read requests

cachefiles: add restore command to recover inflight ondemand read requests

Previously, in ondemand read scenario, if the anonymous fd was closed by
user daemon, inflight and subsequent read requests would return EIO.
As long as the device connection is not released, user daemon can hold
and restore inflight requests by setting the request flag to
CACHEFILES_REQ_NEW.

Suggested-by: Gao Xiang <[email protected]>
Signed-off-by: Jia Zhu <[email protected]>
Signed-off-by: Xin Yin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Jingbo Xu <[email protected]>
Reviewed-by: David Howells <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>