xfs: add a btree entries pointer to struct xfs_da3_icnode_hdr

All but two callers of the ->node_tree_p dir operation already have a
xfs_da3_icnode_hdr from a previous call to xfs_da3_node_hdr_from_disk at
hand.  Add a pointer to the btree entries to struct xfs_da3_icnode_hdr
to clean up this pattern.  The two remaining callers now expand the
whole header as well, but that isn't very expensive and not in a super
hot path anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>