Merge pull request #23770 from Songweibin/wip-rbd-namespace

rbd: correct parameter of namespace and verify it before set_namespace

Reviewed-by: Jason Dillaman <dillaman@redhat.com>

diff --cc src/librbd/librbd.cc
Simple merge

diff --cc src/test/librbd/test_librbd.cc
Simple merge

diff --cc src/tools/rbd/Utils.cc
Simple merge