erasure-code: ErasureCodeJerasure::create_ruleset must return a ruleset

CrushWrapper::add_simple_ruleset does not return a ruleset, it returns
a ruleid that must be converted into a ruleset before being returned.

http://tracker.ceph.com/issues/9044 Fixes: #9044

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