RFR (S): CR 8012939: @Contended doesn't work correctly with inheritance

[Aleksey Shipilev]

Hi,

This is the fix for the bug found during SQE testing:
   http://cr.openjdk.java.net/~shade/8012939/webrev.00/

The essence of the bug is that the padding in the superclass may be
disregarded by the subclasses due to mishap in the instance size
calculation. This is the corner case, and we don't know whether anybody
had been burned on this yet.

The patch fixes up the instance size handling, as well as prunes the
source of the original error (reusing next_nonstatic_type_offset was
confusing).

Testing:
  - full JPRT cycle
  - adhoc testing with Linux x86_64/fastdebug and Test8012939

-Aleksey.