selftests/vm: gup_test: introduce the dump_pages() sub-test

For quite a while, I was doing a quick hack to gup_test.c (previously,
gup_benchmark.c) whenever I wanted to try out my changes to dump_page().
This makes that hack unnecessary, and instead allows anyone to easily get
the same coverage from a user space program.  That saves a lot of time
because you don't have to change the kernel, in order to test different
pages and options.

The new sub-test takes advantage of the existing gup_test infrastructure,
which already provides a simple user space program, some allocated user
space pages, an ioctl call, pinning of those pages (via either
get_user_pages or pin_user_pages) and a corresponding kernel-side test
invocation.  There's not much more required, mainly just a couple of
inputs from the user.

In fact, the new test re-uses the existing command line options in order
to get various helpful combinations (THP or normal, _fast or slow gup, gup
vs.  pup, and more).

New command line options are: which pages to dump, and what type of
"get/pin" to use.

In order to figure out which pages to dump, the logic is:

* If the user doesn't specify anything, the page 0 (the first page in
  the address range that the program sets up for testing) is dumped.

* Or, the user can type up to 8 page indices anywhere on the command
  line.  If you type more than 8, then it uses the first 8 and ignores the
  remaining items.

For example:

    ./gup_test -ct -F 1 0 19 0x1000

Meaning:
    -c:          dump pages sub-test
    -t:          use THP pages
    -F 1:        use pin_user_pages() instead of get_user_pages()
    0 19 0x1000: dump pages 0, 19, and 4096

Link: https://lkml.kernel.org/r/20201026064021.3545418-7-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>