block/009 verify cache dropped about blktests

[osandov]

The first xfs_io pwrite is called with the -w flag, which flushes the data to disk. This initializes the disk to a known state. The second xfs_io pwrite does not have the -w flag, so the data likely is not written to disk yet. This populates the page cache with some testing data.

The first dd/hexdump does not have the direct flag set, so it reads from the page cache. This verifies that the page cache was properly invalidated. The second dd/hexdump uses the direct flag, so it verifies that the disk was discarded.

Hopefully this helps!