ceph: Add check returned value on func ceph_calc_ceph_pg.

Func ceph_calc_ceph_pg maybe failed.So add check for returned value.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>