`zeroSlop` uses `RtsFlags.ParFlags.nCapabilities` whereas it should be using `n_capabilities`

In #22872 I observed a segmentation fault due to zeroSlop seemingly incorrectly zeroing slop in a concurrent program (ghc). This is due to zeroSlop using the following to determine whether it is running in a single-threaded program:

    const bool can_zero_immutable_slop =    
        // Only if we're running single threaded.    
        RTS_DEREF(RtsFlags).ParFlags.nCapabilities <= 1;    

This is wrong as RtsFlags only reflects the arguments set at RTS startup, not how many capabilities are currently active (which may have changed due to setNumCapabilities.

It is remarkable that this went unnoticed for so long.