The smallest disk access for dispersed volumes is 512 bytes. That's fine
when the underlying disk/filesystem uses 512-bytes blocks. However, if
the block size of the bricks is 4KiB, then direct I/O will fail unless
an explicit bigger block size is used.
To fix the test, just increase the block size used for reads.
Updates: #4020
Signed-off-by: Xavi Hernandez <xhernandez@gmail.com>