FS-Cache: Fix cancellation of in-progress operation

Cancellation of an in-progress operation needs to update the relevant counters
and start any operations that are pending waiting on this one.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Steve Dickson <steved@redhat.com>
Acked-by: Jeff Layton <jeff.layton@primarydata.com>