net/mac80211/debugfs.c: prevent build failure with CONFIG_UBSAN=y

With upcoming CONFIG_UBSAN the following BUILD_BUG_ON in
net/mac80211/debugfs.c starts to trigger:

  BUILD_BUG_ON(hw_flag_names[NUM_IEEE80211_HW_FLAGS] != (void *)0x1);

It seems, that compiler instrumentation causes some code
deoptimizations.  Because of that GCC is not being able to resolve
condition in BUILD_BUG_ON() at compile time.

We could make size of hw_flag_names array unspecified and replace the
condition in BUILD_BUG_ON() with following:

  ARRAY_SIZE(hw_flag_names) != NUM_IEEE80211_HW_FLAGS

That will have the same effect as before (adding new flag without
updating array will trigger build failure) except it doesn't fail with
CONFIG_UBSAN.  As a bonus this patch slightly decreases size of
hw_flag_names array.
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/mac80211/debugfs.c

@@ -91,7 +91,7 @@ static const struct file_operations reset_ops = {
 };
 #endif
 
-static const char *hw_flag_names[NUM_IEEE80211_HW_FLAGS + 1] = {
+static const char *hw_flag_names[] = {
 #define FLAG(F)	[IEEE80211_HW_##F] = #F
 	FLAG(HAS_RATE_CONTROL),
 	FLAG(RX_INCLUDES_FCS),
@@ -126,9 +126,6 @@ static const char *hw_flag_names[NUM_IEEE80211_HW_FLAGS + 1] = {
 	FLAG(SUPPORTS_AMSDU_IN_AMPDU),
 	FLAG(BEACON_TX_STATUS),
 	FLAG(NEEDS_UNIQUE_STA_ADDR),
-
-	/* keep last for the build bug below */
-	(void *)0x1
 #undef FLAG
 };
 
@@ -148,7 +145,7 @@ static ssize_t hwflags_read(struct file *file, char __user *user_buf,
 	/* fail compilation if somebody adds or removes
 	 * a flag without updating the name array above
 	 */
-	BUILD_BUG_ON(hw_flag_names[NUM_IEEE80211_HW_FLAGS] != (void *)0x1);
+	BUILD_BUG_ON(ARRAY_SIZE(hw_flag_names) != NUM_IEEE80211_HW_FLAGS);
 
 	for (i = 0; i < NUM_IEEE80211_HW_FLAGS; i++) {
 		if (test_bit(i, local->hw.flags))