btrfs: Remove root argument from cow_file_range_inline

This argument is always set to the root of the inode, which is also
passed. So let's get a reference inside the function and simplify
the arg list.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>