selftests: add hugetlbfstest

As the confusing naming indicates, this test has some overlap with
pre-existing tests.  Would be nice to merge them eventually.  But since it
is only test code, cleanliness is much less important than mere existence.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>