rbd: create+truncate for whole-object layered discards

A whole-object layered discard is implemented as a truncate rather
than a delete: a dummy object is needed to prevent the CoW machinery
from kicking in.  However, a truncate on a non-existent object is
a no-op.  If the object doesn't exist in HEAD, a discard request is
effectively ignored, which violates our "discard zeroes data" promise
and breaks REQ_OP_WRITE_ZEROES implementation.

A non-exclusive create on an existing object is also a no-op, so the
fix is to do a compound create+truncate instead.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>