Merge pull request #20809 from idryomov/wip-librbd-discard-create-truncate

librbd: create+truncate for whole-object layered discards

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

diff --cc src/librbd/io/ObjectRequest.h
Simple merge

diff --cc src/test/librbd/io/test_mock_ObjectRequest.cc
index 496af7fb72bec506e5bc084664d83c6a3dd27269,449f8058c321d9462edc4dfcf0797e190c73a398..27eff02bd95aa716b9d90b1ea2e72fdbe34d1970
--- 1/src/test/librbd/io/test_mock_ObjectRequest.cc
--- 2/src/test/librbd/io/test_mock_ObjectRequest.cc
+++ b/src/test/librbd/io/test_mock_ObjectRequest.cc
@@@ -876,7 -1003,9 +883,8 @@@ TEST_F(TestMockIoObjectRequest, Discard
    InSequence seq;
    expect_get_parent_overlap(mock_image_ctx, CEPH_NOSNAP, 4096, 0);
    expect_prune_parent_extents(mock_image_ctx, {{0, 4096}}, 4096, 4096);
-   expect_object_may_exist(mock_image_ctx, 0, true);
+   expect_object_may_exist(mock_image_ctx, 0, false);
 -  expect_object_map_update(mock_image_ctx, 0, 1, OBJECT_EXISTS, {}, false, 0);
+   expect_create(mock_image_ctx, false);
    expect_truncate(mock_image_ctx, 0, 0);
  
    C_SaferCond ctx;