ceph: just use ci->i_version for fscache aux info

If the i_version regresses, then it's likely that the mtime will do the
same in lockstep with it. There's no need to track both here, just use
the i_version counter since it's just as good and gets the aux size down
to 64 bits.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>