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>