btrfs: move btrfs_extref_hash into inode-item.h

Ideally this would be un-inlined, but that is a cleanup for later.  For
now move this into inode-item.h, which is where the extref code lives.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>