ip: don't export useless error counters for ip6 rewrite

the error node is set to ip6_input in the inline funcition
associated with ip6_rewrite. Thus, error counters defined
for node ip6 rewrite are never used.

Type: fix
Change-Id: Id6bef633928b0fff9069498c2e39e9f5bea2cf9b
Signed-off-by: Arthur de Kerhor <arthurdekerhor@gmail.com>
diff --git a/test/test_mtu.py b/test/test_mtu.py
index 4159deb..ed4fcb5 100644
--- a/test/test_mtu.py
+++ b/test/test_mtu.py
@@ -113,6 +113,8 @@
             # n.show2()
             self.validate_bytes(bytes(p[1]), icmp4_reply)
 
+        self.assert_error_counter_equal("/err/ip4-input/mtu_exceeded", 11)
+
         # Now with DF off. Expect fragments.
         # First go with 1500 byte packets.
         p_payload = UDP(sport=1234, dport=1234) / self.payload(1500 - 20 - 8)
@@ -191,6 +193,8 @@
         for p in rx:
             self.validate_bytes(bytes(p[1]), icmp6_reply_str)
 
+        self.assert_error_counter_equal("/err/ip6-input/mtu_exceeded", 9)
+
         # Reset MTU
         self.vapi.sw_interface_set_mtu(self.pg1.sw_if_index, [current_mtu, 0, 0, 0])