RFR (M): 8151126: Clean up duplicate code for clearing the mark bitmaps

[Thomas Schatzl]

Hi Tom,

  I updated the current webrev with your suggestion.

I also rebased it to tip.

CR:
https://bugs.openjdk.java.net/browse/JDK-8151126
Webrev:
http://cr.openjdk.java.net/~tschatzl/8151126/webrev/
Testing:
jprt

Thanks,
  Thomas