utilities/env_librados: copy use bufferlist::iterator (#6395)

utilities/env_librados: copy use bufferlist::iterator (#6395)

Summary:
to adapt the change in ceph upstream where the bufferlist::copy() method
was removed in
https://github.com/ceph/ceph/commit/c724369010a753bd44e11a534d1f42156c4fc12d

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6395

Differential Revision: D19816815

Pulled By: zhichao-cao

fbshipit-source-id: 9210767b91af0ecdcf5dfaa3e70edcaeea55135f
(cherry picked from commit debc4ef18b2e61581ca81dc7fc997b085eb015d9)