ceph: don't ClearPageChecked in ceph_invalidatepage()

CephFS doesn't set this bit to begin with, so there should be no need
to clear it.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>