rgw: allocate enough space for bucket instance id

Fixes: #8608
Backport: dumpling, firefly
Bucket instance id is a concatenation of zone name, rados instance id,
and a running counter. We need to allocate enough space to account zone
name length.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1977 from ceph/wip-8608

rgw: allocate enough space for bucket instance id

Reviewed-by: Sage Weil <sage@redhat.com>