RFR (XXS): Vm crash "not long aligned" in metaspace stress tests

[Joseph Provino]

Please review this tiny change to one method in one file.

http://cr.openjdk.java.net/~jprovino/8085987/webrev.00

When attempting to specifically allocate from a small chunk,
the size of the request wasn't being rounded up properly.

The fix is to call get_raw_word_size(word_size) and use
that value for the allocation.

Tests:  JPRT, Aurora GC Nightly ad hoc test.