xfs: factor out a xfs_bmap_is_real_extent helper

This checks for all the non-normal extent types, including handling both
encodings of delayed allocations.

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>