Make FP_LOCK_SLOTS_PER_BACKEND look like a function

Make FP_LOCK_SLOTS_PER_BACKEND look like a function

The FP_LOCK_SLOTS_PER_BACKEND macro looks like a constant, but it
depends on the max_locks_per_transaction GUC, and thus can change. This
is non-obvious and confusing, so make it look more like a function by
renaming it to FastPathLockSlotsPerBackend().

While at it, use the macro when initializing fast-path shared memory,
instead of using the formula.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/ffiwtzc6vedo6wb4gbwelon5nefqg675t5c7an2ta7pcz646cg%40qwmkdb3l4ett