selftests/vm: Set mmap()'s fd as -1 for MAP_ANONYMOUS flag

Man page of mmap() says that portable applications should ensure fd
argument to be -1 if MAP_ANONYMOUS flag is set as below:
```
The  mapping is not backed by any file; its contents are initialized to
zero.  The fd and offset arguments are ignored; however, some
implementations require  fd  to be  -1  if  MAP_ANONYMOUS  (or
MAP_ANON)  is specified, and portable applications
should ensure this.
```

However, few mmap() calls under selftests/vm/ uses 0 as fd though they
use MAP_ANONYMOUS flag.  This commit changes the argument to be -1 as
recommended.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>