rgw: fix leak of RGWProcess

Introduced by a3e50b09a1fa22b80dea014d4b7bd96c23904f22.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1005 from ceph/wip-rgw-leak

rgw: fix leak of RGWProcess

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>