Rework locking code in GetMultiXactIdMembers

Rework locking code in GetMultiXactIdMembers

After commit 53c2a97a9266, the code flow around the "retry" goto label
in GetMultiXactIdMembers was confused about what was possible: we never
return there with a held lock, so there's no point in testing for one.
This realization lets us simplify the code a bit.  While at it, make the
scope of a couple of local variables in the same function a bit tighter.

Per Coverity.