btrfs: make btrfs_check_data_free_space take btrfs_inode

Instead of calling BTRFS_I on the passed vfs_inode take btrfs_inode
directly.

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