generic: Test races between mmap and normal write for prefaulted ranges

generic: Test races between mmap and normal write for prefaulted ranges

Add test which spawns two threads both writing one file via mmap which
has been previously prefaulted by reading. Do the same test when one
thread accesses the file via mmap and the other one via normal write.
This is mainly interesting to uncover races in DAX fault handling and
between DAX fault handling and write path.

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>