xfs: fix xfs_get_extsz_hint for a zero extent size hint

We can easily set the extsize flag without setting an extent size
hint, or one that evaluates to zero.  Historically the di_extsize
field was only used when it was non-zero, but the commit

	"Cleanup inode extent size hint extraction"

broke this.  Restore the old behaviour, thus fixing xfsqa 090 with
a debug kernel.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>