btrfs: sink parameter is_data to btrfs_set_disk_extent_flags

The parameter has been added in 2009 in the infamous monster commit
5d4f98a2 ("Btrfs: Mixed back reference  (FORWARD ROLLING FORMAT
CHANGE)") but not used ever since. We can sink it and allow further
simplifications.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>