cachefiles: flush all requests after setting CACHEFILES_DEAD

In the Linux kernel, the following vulnerability has been resolved:

cachefiles: flush all requests after setting CACHEFILES_DEAD

In ondemand mode, when the daemon is processing an open request, if the

kernel flags the cache as CACHEFILES_DEAD, the cachefiles_daemon_write()

will always return -EIO, so the daemon can't pass the copen to the kernel.

Then the kernel process that is waiting for the copen triggers a hung_task.

Since the DEAD state is irreversible, it can only be exited by closing

/dev/cachefiles. Therefore, after calling cachefiles_io_error() to mark

the cache as CACHEFILES_DEAD, if in ondemand mode, flush all requests to

avoid the above hungtask. We may still be able to read some of the cached

data before closing the fd of /dev/cachefiles.

Note that this relies on the patch that adds reference counting to the req,

otherwise it may UAF.