erasure-code: ErasureCodeInterface::get_chunk_mapping()

erasure-code: ErasureCodeInterface::get_chunk_mapping()

Add support for erasure code plugins that do not sequentially map the
chunks encoded to the corresponding index. This is mostly transparent to
the caller, except when it comes to retrieving the data chunks when
reading. For this purpose there needs to be a remapping function so the
caller has a way to figure out which chunks actually contain the data
and reorder them.

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