fscache: Implement simple cookie state machine

fscache: Implement simple cookie state machine

Implement a very simple cookie state machine to handle lookup, withdrawal,
relinquishment and, eventually, timed committing and invalidation.

Three cache methods are provided: ->lookup_cookie() to look up and, if
necessary, create a data storage object; ->withdraw_cookie() to free the
resources associated with that object and potentially delete it; and
->prepare_to_write(), to do prepare for changes to the cached data to be
modified locally.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com