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

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

Signed-off-by: Loic Dachary <loic@dachary.org>
osd: improve ruleno/ruleset consistency

rename variables to reflect when they hold a ruleset and when they hold
a rule id / ruleno in the hope to avoid bugs in the future.

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