tools/testing/selftests/vm/: add MAP_POPULATE test

As with many other projects, we use some shmalloc allocator.  At some
point we need to make a part of allocated pages back private to process.
And it should be populated straight away.  Check that (MAP_PRIVATE |
MAP_POPULATE) actually copies the private page.

[akpm@linux-foundation.org: change message, per review discussion]
Link: http://lkml.kernel.org/r/20180801233636.29354-1-dima@arista.com


Signed-off-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Hua Zhong <hzhong@arista.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Stuart Ritchie <sritchie@arista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>