ceph: allow idmapped atomic_open inode op

Enable ceph_atomic_open() to handle idmapped mounts. This is just a
matter of passing down the mount's idmapping.

[ aleksandr.mikhalitsyn: adapted to 5fadbd99 ("ceph: rely on vfs for
  setgid stripping") ]

Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>