cls/refcount: ENOENT when put on non-existent object

cls/refcount: ENOENT when put on non-existent object

If we get ENOENT, do not that that to mean an implicit reference count of
1.  That means that if you put a non-existent object, we should get
ENOENT instead of doing a useless delete on the OSD.

Note that this changes the get behavior slightly, too: doing a get on a
non-existent object will now fail with ENOENT instead of implicitly
creating it.

Stumbled across this from #10262.

Signed-off-by: Sage Weil <sage@redhat.com>