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>
You need to call the other decode if the chunks you have are not all the data chunks. This decode function simply concats the chunks. You need the decode function that rebuilds the missing chunks.