mmap/mmap12.c: new case to test MAP_POPULATE of mmap

Create test case for MAP_POPULATE of mmap.

Verify the following statement
"For a file mapping,this causes read-ahead on the file.
 Later accesses to the mapping will not be blocked by page faults."
by mmap-ing a file and check if all pages are present.

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>