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>
Right. It makes me wonder : shouldn't this problem (i.e. not being able to read the required amount of bytes) be caught earlier ? That is, when the OSD that has the chunk fails to read the expected bytes, it could return an error instead of returning an invalid number of bytes. What do you think ?