Figure out what read path count enforcement should look like.

[bzbarsky-apple]

Problem

Right now (except in configurations that allow completely unlimited paths) we outright reject any read attempt that has more than the spec-mandated minimum number of paths, no matter how many paths we can actually support or how many fabrics we actually have configured.

Proposed Solution

Figure out whether we can have a maximum that is not also the bare minimum, possible depending on device configuration.

[mrjerryjohns]

@erjiaqing assigning this to you for now since this is your wheel-house.