btrfs: move all btree inode initialization into btrfs_init_btree_inode

Move the remaining code that deals with initializing the btree
inode into btrfs_init_btree_inode instead of splitting it between
that helpers and its only caller.

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