buffers: make buffer data size configurable from startup config

Example:

buffers {
	default data-size 1536
}

Change-Id: I5b4436850ca18025c9fdcfc7ed648c2c2732d660
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vnet/dhcp/dhcp6_proxy_node.c b/src/vnet/dhcp/dhcp6_proxy_node.c
index 92e011b..e4464e0 100644
--- a/src/vnet/dhcp/dhcp6_proxy_node.c
+++ b/src/vnet/dhcp/dhcp6_proxy_node.c
@@ -316,7 +316,7 @@
 	link_address_set:
 
 	  if ((b0->current_length + sizeof (*id1) + sizeof (*vss1) +
-	       sizeof (*cmac)) > VLIB_BUFFER_DATA_SIZE)
+	       sizeof (*cmac)) > vlib_bufer_get_default_size (vm))
 	    {
 	      error0 = DHCPV6_PROXY_ERROR_PKT_TOO_BIG;
 	      next0 = DHCPV6_PROXY_TO_SERVER_INPUT_NEXT_DROP;