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

fs/xfs/xfs_rw.c

@@ -173,17 +173,9 @@ xfs_extlen_t
 xfs_get_extsz_hint(
 	struct xfs_inode	*ip)
 {
-	xfs_extlen_t		extsz;
-
-	if (unlikely(XFS_IS_REALTIME_INODE(ip))) {
-		extsz = (ip->i_d.di_flags & XFS_DIFLAG_EXTSIZE)
-				? ip->i_d.di_extsize
-				: ip->i_mount->m_sb.sb_rextsize;
-		ASSERT(extsz);
-	} else {
-		extsz = (ip->i_d.di_flags & XFS_DIFLAG_EXTSIZE)
-				? ip->i_d.di_extsize : 0;
-	}
-
-	return extsz;
+	if ((ip->i_d.di_flags & XFS_DIFLAG_EXTSIZE) && ip->i_d.di_extsize)
+		return ip->i_d.di_extsize;
+	if (XFS_IS_REALTIME_INODE(ip))
+		return ip->i_mount->m_sb.sb_rextsize;
+	return 0;
 }