mm/gup_benchmark: fix MAP_HUGETLB case

The MAP_HUGETLB ("-H" option) of gup_benchmark fails:

  $ sudo ./gup_benchmark -H
  mmap: Invalid argument

This is because gup_benchmark.c is passing in a file descriptor to
mmap(), but the fd came from opening up the /dev/zero file.  This
confuses the mmap syscall implementation, which thinks that, if the
caller did not specify MAP_ANONYMOUS, then the file must be a huge page
file.  So it attempts to verify that the file really is a huge page
file, as you can see here:

ksys_mmap_pgoff()
{
    if (!(flags & MAP_ANONYMOUS)) {
        retval = -EINVAL;
        if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
            goto out_fput; /* THIS IS WHERE WE END UP */

    else if (flags & MAP_HUGETLB) {
        ...proceed normally, /dev/zero is ok here...

...and of course is_file_hugepages() returns "false" for the /dev/zero
file.

The problem is that the user space program, gup_benchmark.c, really just
wants anonymous memory here.  The simplest way to get that is to pass
MAP_ANONYMOUS whenever MAP_HUGETLB is specified, so that's what this
patch does.

Link: http://lkml.kernel.org/r/20191021212435.398153-2-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>