ip: fix assert in ip4_ttl_inc

There is no need to verify the checksum for packets that have the IP
checksum offload flag set. This uses the same logic as
ip4_ttl_and_checksum_check.

Type: fix

Signed-off-by: Aloys Augustin <aloaugus@cisco.com>
Change-Id: I177b07212a992362a4c965c074dcecf1e504c593
diff --git a/src/vnet/ip/ip4_forward.c b/src/vnet/ip/ip4_forward.c
index 3c67bb3..3556d35 100644
--- a/src/vnet/ip/ip4_forward.c
+++ b/src/vnet/ip/ip4_forward.c
@@ -2055,7 +2055,9 @@
   ttl += 1;
   ip->ttl = ttl;
 
-  ASSERT (ip4_header_checksum_is_valid (ip));
+  ASSERT (ip4_header_checksum_is_valid (ip) ||
+	  (vnet_buffer (b)->oflags & VNET_BUFFER_OFFLOAD_F_IP_CKSUM) ||
+	  (vnet_buffer (b)->oflags & VNET_BUFFER_OFFLOAD_F_OUTER_IP_CKSUM));
 }
 
 /* Decrement TTL & update checksum.