btrfs: remove the wait argument to btrfs_start_ordered_extent

Given that wait is always set to 1, so remove the argument.
Last use of wait with 0 was in 0c304304 ("Btrfs: remove
csum_bytes_left").

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>