ceph: conversion to new fscache API

Now that the fscache API has been reworked and simplified, change ceph
over to use it.

With the old API, we would only instantiate a cookie when the file was
open for reads. Change it to instantiate the cookie when the inode is
instantiated and call use/unuse when the file is opened/closed.

Also, ensure we resize the cached data on truncates, and invalidate the
cache in response to the appropriate events. This will allow us to
plumb in write support later.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20211129162907.149445-2-jlayton@kernel.org/ # v1
Link: https://lore.kernel.org/r/20211207134451.66296-2-jlayton@kernel.org/ # v2
Link: https://lore.kernel.org/r/163906984277.143852.14697110691303589000.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967188351.1823006.5065634844099079351.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021581427.640689.14128682147127509264.stgit@warthog.procyon.org.uk/ # v4