ECBackend: Using ROUND_UP_TO to refactor function get_recovery_chunk_size()

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Merge pull request #2183 from majianpeng/master

ECBackend: Using ROUND_UP_TO to refactor function get_recovery_chunk_size()

Reviewed-by: Samuel Just <sam.just@inktank.com>