cachefiles: Set object to close if ondemand_id < 0 in copen

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

cachefiles: Set object to close if ondemand_id < 0 in copen

If copen is maliciously called in the user mode, it may delete the request

corresponding to the random id. And the request may have not been read yet.

Note that when the object is set to reopen, the open request will be done

with the still reopen state in above case. As a result, the request

corresponding to this object is always skipped in select_req function, so

the read request is never completed and blocks other process.

Fix this issue by simply set object to close if its id < 0 in copen.