rgw: remove memory allocation

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: get rid of a memory allocation

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1515 from ceph/wip-rgw-less-alloc

Wip rgw less alloc

Reviewed-by: Sage Weil <sage@inktank.com>