ceph: disable updating the atime since cephfs won't maintain it

Since CephFS makes no attempt to maintain atime, we shouldn't
try to update it in mmap and generic read cases and ignore updating
it in direct and sync read cases.

And even we update it in mmap and generic read cases we will drop
it and won't sync it to MDS. And we are seeing the atime will be
updated and then dropped to the floor again and again.

URL: https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/VSJM7T4CS5TDRFF6XFPIYMHP75K73PZ6/


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>