[Patch] Make MALLOC_ARENA_{TEST,MAX} implementation match documentation

[Jeff Law]

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The NEWS file with glibc-2.10 implies that when MALLOC_ARENA_TEST is
set, the test for the number of cores is only performed once the
number of arenas in use reaches the value specified by
MALLOC_ARENA_TEST.  This is done because of the cost of testing for
the number of cores.

However, that is not actually what the code in arena.c implements.

Within arena_get2 we have the following code:

  static size_t narenas_limit;

  a = get_free_list ();
  if (a == NULL)
    {
      /* Nothing immediately available, so generate a new arena.  */
      if (narenas_limit == 0)			/* TEST 1 */
        {
          if (mp_.arena_max != 0)		/* TEST 2 */
            narenas_limit = mp_.arena_max;
          else
            {
              int n  = __get_nprocs ();

              if (n >= 1)
                narenas_limit = NARENAS_FROM_NCORES (n);
              else
                /* We have no information about the system.  Assume two
                   cores.  */
                narenas_limit = NARENAS_FROM_NCORES (2);
            }
        }

Assume we have ARENA_MAX=0 ARENA_TEST=10.


The first time through this code narenas_limit will be zero and
mp_.arena_max will be zero as well.  TEST 1 will be true & TEST 2
will be false, resulting in execution of the ELSE clause which calls
__get_nprocs regardless of the value of ARENA_TEST.

The second issue that we've spotted is that ARENA_MAX is supposed to
be an upper limit on the number of arenas.  But if ARENA_MAX is <=
ARENA_TEST, then we will create up to ARENA_TEST arenas.  The problem
is we test narenas < arena_test || narenas < arena_max...  This is
easily resolved by clamping the value of arenas_test based on the
value of arenas_max.

Fixes for both problems are attached.
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