osd_types: fix object_info_t backwards compatibility

Shipping an object_info_t to a replica with the dirty
flag set would cause the replica to interpret that
object as being lost.  Instead, we always encode
lost into the slot where dumpling expects to find
it and add another field at the end of the encoding.

Backport: emperor
Fixes: #6761
Signed-off-by: Samuel Just <sam.just@inktank.com>