btrfs/26[67]: update the stale comments

btrfs/26[67]: update the stale comments

Test case btrfs/266 is verifying the buffered read repair on RAID1C3,
and btrfs/267 is verifying the direct IO read repair on RAID1C3.

However btrfs/267 is using incorrect comments, it says the test case is
verify buffered read repair, but it's not the case.

Fix those stale comments by explicitly mention buffered/direct IO for
each test case. (btrfs/266 for buffered, btrfs/267 for direct)

Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>