datapath: compat: Fix compilation issue with grsecurity patch

Grsecurity patch enables GCC's constify plugin so that it will
automatically constify a class of type that contains only function
pointers.  However, if the type is also specified by __read_mostly, it
will put the constify object into the read_mostly section that results
in compilation error.  This patch works around the compilation issue by
disabling __ready_mostly when grsecurity patch is applied.

Tested with 4.14.33 kernel with grsecurity patch.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>

datapath/compat.h

@@ -28,6 +28,13 @@
 #include <net/netfilter/ipv6/nf_defrag_ipv6.h>
 #include <net/netfilter/nf_conntrack_count.h>
 
+/* Fix grsecurity patch compilation issue. */
+#ifdef CONSTIFY_PLUGIN
+#include <linux/cache.h>
+#undef __read_mostly
+#define __read_mostly
+#endif
+
 /* Even though vanilla 3.10 kernel has grp->id, RHEL 7 kernel is missing
  * this field. */
 #ifdef HAVE_GENL_MULTICAST_GROUP_WITH_ID