rt_dsfield: fix Expedited Forwarding PHB

RFC 2598 defines Expedited Forwarding in section 2.3
   Codepoint 101110 is recommended for the EF PHB.
which translates to B8 as encoded in rt_dsfield

etc/iproute2/rt_dsfield

@@ -22,4 +22,5 @@
 0xA0	CS5
 0xC0	CS6
 0xE0	CS7
-0x5C	EF
+# RFC 2598
+0xB8	EF