btrfs: fix comment for btrfs ordered extent flag bits

There is small error in comment about BTRFS_ORDERED_* flags, added in
commit 3c198fe0 ("btrfs: rework the order of
btrfs_ordered_extent::flags") but the fixup did not get merged in time.

The 4 types are for ordered extent itself, not for direct io.
Only 3 types support direct io, REGULAR/NOCOW/PREALLOC.

Fix the comment to reflect that.
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ordered-data.h

@@ -39,8 +39,8 @@ struct btrfs_ordered_sum {
  */
 enum {
 	/*
-	 * Different types for direct io, one and only one of the 4 type can
-	 * be set when creating ordered extent.
+	 * Different types for ordered extents, one and only one of the 4 types
+	 * need to be set when creating ordered extent.
 	 *
 	 * REGULAR:	For regular non-compressed COW write
 	 * NOCOW:	For NOCOW write into existing non-hole extent