selftests: expanding more mlock selftest

This patch will randomly perform mlock/mlock2 on a given memory region,
and verify the RLIMIT_MEMLOCK limitation works properly.

Suggested-by: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/1473325970-11393-4-git-send-email-wei.guo.simon@gmail.com
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Eric B Munson <emunson@akamai.com>
Cc: Simon Guo <wei.guo.simon@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Alexey Klimov <klimov.linux@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>