erasure code: backend feature to read from all OSDs and silently discard EIO if possible

Bug: http://tracker.ceph.com/issues/9943

Brief: Add osd setting osd_pool_erasure_code_subread_all. Default is false. If set it to true. Will send sub-read op to all OSD and choose the first success read result.

Signed-off-by: Wei Luo <luowei@yahoo-inc.com>
In which situation would it be ok to have a chunk that has a length different from ec->sinfo.get_chunk_size() ? In other words, why is it not ok to replace the above line with:
<pre>
if ((data_len == ec->sinfo.get_chunk_size()) 
</pre>