btrfs: move the auto defrag code to defrag.c

This currently exists in file.c, move it to the more natural location in
defrag.c.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ reformat comments ]
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>