RFR (XS): 8155721: Sparse remset wastes half of entry memory

[Thomas Schatzl]

Hi all,

  can I have reviews for this pretty small change that (basically)
halves memory usage of the sparse prt?

Looking through the code showed that G1 allocates
capacity SparsePRTEntrys (the container for the cards for a particular
region), but already expands the hash table at capacity/2+1. So almost
half of these SparsePRTEntrys are never used and need not be
allocated/cleared/etc.

This also uncovered a off-by-one bug in RSHashTable::alloc_entry() that
was benign before but caused crashes when allocating just what is
required.

CR:
https://bugs.openjdk.java.net/browse/JDK-8155721
Webrev:
http://cr.openjdk.java.net/~tschatzl/8155721/webrev/jprt, gcbasher with -XX:+VerifyRememberedSet

Thanks,
  Thomas