fscache: Allow cancelled operations to be enqueued

fscache: Allow cancelled operations to be enqueued

BugLink: https://bugs.launchpad.net/bugs/1774336
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: 9ae326a69004 ("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>
(cherry picked from commit d0eb06afe712b7b103b6361f40a9a0c638524669)
Signed-off-by: Daniel Axtens <daniel.axtens@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>