compat: Detect and use inet_frag_queue->last_in.

Kernels 3.17 and older have this field, while newer kernels use the
'flags' field. Detect this in the build in case anyone backports this
change to an older kernel.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>

acinclude.m4

@@ -353,6 +353,9 @@ AC_DEFUN([OVS_CHECK_LINUX_COMPAT], [
   OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [ip_skb_dst_mtu])
   OVS_GREP_IFELSE([$KSRC/include/net/inet_frag.h], [hashfn.*const],
                   [OVS_DEFINE([HAVE_INET_FRAGS_CONST])])
+  OVS_GREP_IFELSE([$KSRC/include/net/inet_frag.h], [last_in],
+                  [OVS_DEFINE([HAVE_INET_FRAGS_LAST_IN])])
+
   OVS_GREP_IFELSE([$KSRC/include/net/dst_metadata.h], [metadata_dst])
 
   OVS_GREP_IFELSE([$KSRC/include/linux/net.h], [sock_create_kern.*net],

datapath/linux/compat/include/net/inet_frag.h

@@ -14,7 +14,7 @@
 #endif
 
 #ifdef OVS_FRAGMENT_BACKPORT
-#if LINUX_VERSION_CODE < KERNEL_VERSION(3,17,0)
+#ifdef HAVE_INET_FRAGS_LAST_IN
 #define q_flags(q) (q->last_in)
 #define qp_flags(qp) (qp->q.last_in)
 #else