Update `cdroot' disktab entry to match the volume header size we use now,

and mention this in the Makefile so that this hopefully doesn't get broken
next time the size is modified. Repairs install45.iso.

src/etc/etc.sgi/disktab

@@ -35,8 +35,8 @@ rdroot|ramdiskroot|RAM-disk root FS image:\
 
 cdroot|cdramdiskroot|RAM-disk root FS image:\
 	:ty=simulated:se#2048:nc#16:nt#224:ns#32:\
-	:ta=4.2BSD:oa#22:pa#114666:fa#2048:ba#16384:\
-	:ob#0:pb#0:oc#0:pc#114688:op#0:pp#22:
+	:ta=4.2BSD:oa#80:pa#114608:fa#2048:ba#16384:\
+	:ob#0:pb#0:oc#0:pc#114688:op#0:pp#80:
 
 # Seagate ST31200N
 ST31200N-MBR:\