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>
It is indeed required in all cases, you are correct. In the case of !ec->subread_all it should either assert or return on error because there is no way for it to recover at this point. If ec->subread_all the situation is different because we may end up with enough chunks to complete the read.