erasure-code: delegate chunk remapping to the plugin

erasure-code: delegate chunk remapping to the plugin

Remapping the chunks after they have been encoded by the plugin is fine
if remapping is transparent to the plugin. However, the goal is to
delegate the remapping of the chunks to the plugin, for LRC in
particular. The chunks are therefore reordered according to the
directions found in the mapping array before they are passed to
decode_chunks.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>