erasure-code: Avoid k/m that we set are not equal in erasure-plugin.

Because erasure-plugin has default k/m and can autotune if k or m
invalid. Check k/m they are the same as we want.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Merge pull request #2305 from majianpeng/fix5

Make ceph_erasure_code_benchmark more robust.

Reviewed-by: Loic Dachary <loic-201408@dachary.org>