btrfs: pass a btrfs_inode to is_data_inode()

Pass a struct btrfs_inode to is_data_inode() as it's an
internal interface, allowing to remove some use of BTRFS_I.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>