librbd: rename howmany to avoid conflict

A howmany macro exists on some platforms in standard headers, but there
really isn't any sort of standard that I've found. We just avoid the
conflict entirely this way.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
TrackedOp: remove unused private variable

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
librbd: remove unused private variable

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>