generic: test which tries to exercise AIO/DIO into unwritten space

generic: test which tries to exercise AIO/DIO into unwritten space

This test verifies that the an unwritten extent is properly marked as
written after writing into it.

There was a hard-to-hit bug which would occasionally trigger with ext4
for which this test was a reproducer.  This has been fixed after
moving ext4 to use iomap for Direct I/O's, although as of this
writing, there are still some occasional failures on ext4 when block
size < page size.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>