ofp-actions: Make struct ofpact constant size across implementations.

Before commit c2d936a44f (ofp-actions: Centralize all OpenFlow action
code for maintainability.), struct ofpact was 4 bytes with GCC and Clang,
and 12 bytes with other compilers.  That commit changed struct ofpact so
that it remained 4 bytes with GCC and Clang but shrank to 8 bytes on other
compilers.  An unexpected side effect of that change was that the size
of the pad[] array in struct ofpact_nest shrank to 0 bytes, because that
array padded to a multiple of 8 bytes.  MSVC does not support 0-element
arrays, so this caused a build failure.

This commit fixes the problem by changing struct ofpact so that it is 4
bytes with every compiler.

Reported-by: Alin Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alin Serdean <aserdean@cloudbasesolutions.com>

lib/compiler.h

@@ -169,6 +169,7 @@
  * or otherwise exposed outside of a single process. */
 #if __GNUC__ && !__CHECKER__
 #define OVS_PACKED_ENUM __attribute__((__packed__))
+#define HAVE_PACKED_ENUM
 #else
 #define OVS_PACKED_ENUM
 #endif

lib/ofp-actions.h

@@ -157,16 +157,20 @@ enum {
  *       code to translate the ofpact to OpenFlow must tolerate this case.)
  */
 struct ofpact {
+    /* We want the space advantage of an 8-bit type here on every
+     * implementation, without giving up the advantage of having a useful type
+     * on implementations that support packed enums. */
+#ifdef HAVE_PACKED_ENUM
     enum ofpact_type type;      /* OFPACT_*. */
+#else
+    uint8_t type;               /* OFPACT_* */
+#endif
+
     uint8_t raw;                /* Original type when added, if any. */
     uint16_t len;               /* Length of the action, in bytes, including
                                  * struct ofpact, excluding padding. */
 };
-
-#ifdef __GNUC__
-/* Make sure that OVS_PACKED_ENUM really worked. */
 BUILD_ASSERT_DECL(sizeof(struct ofpact) == 4);
-#endif
 
 /* Alignment. */
 #define OFPACT_ALIGNTO 8