selftests/vm: hmm-tests: remove the libhugetlbfs dependency

HMM selftests are incredibly useful, but they are only effective if people
actually build and run them.  All the other tests in selftests/vm can be
built with very standard, always-available libraries: libpthread, librt.
The hmm-tests.c program, on the other hand, requires something that is
(much) less readily available: libhugetlbfs.  And so the build will
typically fail for many developers.

A simple attempt to install libhugetlbfs will also run into complications
on some common distros these days: Fedora and Arch Linux (yes, Arch AUR
has it, but that's fragile, as always with AUR).  The library is not
maintained actively enough at the moment, for distros to deal with it.  I
had to build it from source, for Fedora, and that didn't go too smoothly
either.

It turns out that, out of 21 tests in hmm-tests.c, only 2 actually require
functionality from libhugetlbfs.  Therefore, if libhugetlbfs is missing,
simply ifdef those two tests out and allow the developer to at least have
the other 19 tests, if they don't want to pause to work through the above
issues.  Also issue a warning, so that it's clear that there is an
imperfection in the build.

In order to do that, a tiny shell script (check_config.sh) runs a quick
compile (not link, that's too prone to false failures with library paths),
and basically, if the compiler doesn't find hugetlbfs.h in its standard
locations, then the script concludes that libhugetlbfs is not available.
The output is in two files, one for inclusion in hmm-test.c
(local_config.h), and one for inclusion in the Makefile (local_config.mk).

Link: https://lkml.kernel.org/r/20201026064021.3545418-9-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>