[libmultipath] don't re-select the getprio fn when not needed

[libmultipath] don't re-select the getprio fn when not needed

The pp->getprio was used
1) to store the prioritizer function pointer
2) to determine if the prioritizer was already selected

But NULL is a valid value for already selected getprio.
Which causes uneeded re-selection.

So introduce a pp->getprio_selected flag to straight this out.

Edward Goggin, EMC