rgw: set a default data extra pool name

Fixes: #8585
Have a default name for the data extra pool, otherwise it would be empty
which means that it'd default to the data pool name (which is a problem
with ec backends).

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1955 from ceph/wip-8585

rgw: set a default data extra pool name

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>