librbd: make C++ cmp&write semantics equal to C API

librbd: make C++ cmp&write semantics equal to C API

The compare and write C++ API (both sync and async) does not check
compare bufferlist length and executes compare ops of bufferlist length
size, even if (write) "len" is smaller than bufferlist length.
This patch changes this behavior by always issuing compares and
writes of "len" size to match the C API. The bufferlist length
can be greater than "len" for both compare and write but only
"len" bytes are compared and written. If the bufferlist length
is smaller than "len" the call will fail.

Signed-off-by: Jonas Pfefferle <pepperjo@japf.ch>