nptl: Correct stack size attribute when stack grows up [BZ #32574]

nptl: Correct stack size attribute when stack grows up [BZ #32574]

Set stack size attribute to the size of the mmap'd region only
when the size of the remaining stack space is less than the size
of the mmap'd region.

This was reversed.  As a result, the initial stack size was only
135168 bytes.  On architectures where the stack grows down, the
initial stack size is approximately 8384512 bytes with the default
rlimit settings.  The small main stack size on hppa broke
applications like ruby that check for stack overflows.

Signed-off-by: John David Anglin <dave.anglin@bell.net>