Merge pull request #4186 from ceph/wip-7787

librbd: add fast diff feature

Reviewed-by: Josh Durgin <jdurgin@redhat.com>

diff --cc doc/man/8/rbd.rst
Simple merge

diff --cc src/cls/rbd/cls_rbd.cc
Simple merge

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

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

diff --cc src/rbd.cc
Simple merge

diff --cc src/test/cli/rbd/help.t
Simple merge

diff --cc src/test/cls_rbd/test_cls_rbd.cc
Simple merge