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>
(cherry picked from commit b2542f8e880e45bb6d052f13eb3ef6a7c61e4cd4)