generic: truncate after a mix of dio and buffered writes

generic: truncate after a mix of dio and buffered writes

Test that if we have a file with a hole, do a mix of direct IO and
buffered writes to it and truncate the file to a size that lies in
the middle of the hole, after unmounting and mounting again the
filesystem, the file has a correct size and no data loss happened.

This test is motivated by a bug found in btrfs when used with the
no-holes feature (i.e. MKFS_OPTIONS="-O no-holes") which is fixed by
the following patch for the linux kernel:

  Btrfs: fix data loss after truncate when using the no-holes feature

[eguan: add _require_odirect]

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>