btrfs, mm: remove the punt_to_cgroup field in struct writeback_control

punt_to_cgroup is only used by extent_write_locked_range, but that
function also directly controls the bio flags for the actual submission.
Remove th punt_to_cgroup field, and just set REQ_CGROUP_PUNT directly
in extent_write_locked_range.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>