Btrfs: specify a new ordered extent type for create_io_em

As 0 refers to an existing type BTRFS_ORDERED_IO_DONE, this specifies a
new type 'REGULAR' for regular IO.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>