selftests/vm: add test for mlock() when areas are intersected

This patch adds mlock() test for multiple invocation on the same address
area, and verify it doesn't mess the rlimit mlock limitation.

Link: http://lkml.kernel.org/r/1472554781-9835-5-git-send-email-wei.guo.simon@gmail.com
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Cc: Alexey Klimov <klimov.linux@gmail.com>
Cc: Eric B Munson <emunson@akamai.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Simon Guo <wei.guo.simon@gmail.com>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>