generic: test pagecache invalidation after direct write

generic: test pagecache invalidation after direct write

Test if direct write invalidates pagecache correctly, so that
subsequent buffer read reads the correct data from disk.

This test is inspired by LTP tests dio29, and serves as a regression
test for the bug found by it, see kernel commit c771c14baa33
("iomap: invalidate page caches should be after iomap_dio_complete()
in direct write").

The test can be easily expanded to other write/read combinations,
e.g.  buffer write + direct read and direct write + direct read, so
they are also being tested.

Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>