Fix EINVAL err when use "ceph tell osd.* bench"

  Signed-off-by: huangjun <hjwsm1989@gmail.com>
Merge pull request #1974 from hjwsm1989/osd-bench

  Fix EINVAL err when use "ceph tell osd.* bench"

Reviewed-by: Loic Dachary <loic@dachary.org>