qsort: Fix a typo causing unnecessary malloc/free (BZ 31276)

In qsort_r we allocate a buffer sized QSORT_STACK_SIZE (1024) on stack
and we intend to use it if all elements can fit into it.  But there is a
typo:

    if (total_size < sizeof buf)
      buf = tmp;
    else
      /* allocate a buffer on heap and use it ... */

Here "buf" is a pointer, thus sizeof buf is just 4 or 8, instead of
1024.  There is also a minor issue that we should use "<=" instead of
"<".

This bug is detected debugging some strange heap corruption running the
Ruby-3.3.0 test suite (on an experimental Linux From Scratch build using
Binutils-2.41.90 and Glibc trunk, and also Fedora Rawhide [1]).  It
seems Ruby is doing some wild "optimization" by jumping into somewhere
in qsort_r instead of calling it normally, resulting in a double free of
buf if we allocate it on heap.  The issue can be reproduced
deterministically with:

    LD_PRELOAD=/usr/lib/libc_malloc_debug.so MALLOC_CHECK_=3 \
    LD_LIBRARY_PATH=. ./ruby test/runner.rb test/ruby/test_enum.rb

in Ruby-3.3.0 tree after building it.  This change would hide the issue
for Ruby, but Ruby is likely still buggy (if using this "optimization"
sorting larger arrays).

[1]:https://kojipkgs.fedoraproject.org/work/tasks/9729/111889729/build.log



Signed-off-by: Xi Ruoyao <xry111@xry111.site>