btrfs: make need_force_cow take btrfs_inode

Gets rid of superfulous BTRFS_I() calls and prepare for converting
btrfs_run_delalloc_range to using btrfs_inode.

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