btrfs: move writeback_control::nr_to_write update to __extent_writepage

Move the nr_to_write accounting from __extent_writepage_io to
__extent_writepage_io as we'll grow another __extent_writepage_io that
doesn't want this accounting soon.  Also drop the obsolete comment -
decrementing a counter in the on-stack writeback_control data structure
doesn't need the page lock.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>