Fix/2.6/rb6051

Messages are put on a pnfs manager queue by pnfs id or path. This is to
ensure that we don't perform concurrent operations on a name space
object and two enforce a happens-before relationship on operations on
a name space object (ie to avoid messages from being processed out of
order). It is also essential for request folding to work correctly.

PnfsCreateEntryMessage and PnfsMapPathMessage however use their own path
field rather than the one in the PnfsMessage base class. Therefore a
random queue was chosen for these messages.

This patch fixes this by also binding the path in the base class. A future
patch for master should remove the redudant fields.