fscache: Allow cancelled operations to be enqueued

Alter the state-check assertion in fscache_enqueue_operation() to allow
cancelled operations to be given processing time so they can be cleaned up.

Also fix a debugging statement that was requiring such operations to have
an object assigned.

Fixes: 9ae326a6 ("CacheFiles: A cache that backs onto a mounted filesystem")
Reported-by: Kiran Kumar Modukuri <kiran.modukuri@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>