fix the bug if we set pgp_num=-1 using "ceph osd pool set data|metadata|rbd -1"
   will set the pgp_num to a hunge number.

   Signed-off-by: huangjun  <hjwsm1989@gmail.com>
(cherry picked from commit bf198e673fd876e34006d3c83f0479454e6295aa)