erasure-code: make decode_concat virtual

So that a plugin can provide a more efficient implementation.

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>
Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1901 from dachary/wip-erasure-code-concat

erasure-code: make decode_concat virtual

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>