btrfs: open code btrfs_csum_ptr

Remove btrfs_csum_ptr() and fold it into it's only caller.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>