compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

Reserve the pseudo keyword 'fallthrough' for the ability to convert the
various case block /* fallthrough */ style comments to appear to be an
actual reserved word with the same gcc case block missing fallthrough
warning capability.

All switch/case blocks now should end in one of:

	break;
	fallthrough;
	goto <label>;
	return [expression];
	continue;

In C mode, GCC supports the __fallthrough__ attribute since 7.1,
the same time the warning and the comment parsing were introduced.

fallthrough devolves to an empty "do {} while (0)" if the compiler
version (any version less than gcc 7) does not support the attribute.
Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler_attributes.h

@@ -40,6 +40,7 @@
 # define __GCC4_has_attribute___noclone__             1
 # define __GCC4_has_attribute___nonstring__           0
 # define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
+# define __GCC4_has_attribute___fallthrough__         0
 #endif
 
 /*
@@ -185,6 +186,22 @@
 # define __noclone
 #endif
 
+/*
+ * Add the pseudo keyword 'fallthrough' so case statement blocks
+ * must end with any of these keywords:
+ *   break;
+ *   fallthrough;
+ *   goto <label>;
+ *   return [expression];
+ *
+ *  gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
+ */
+#if __has_attribute(__fallthrough__)
+# define fallthrough                    __attribute__((__fallthrough__))
+#else
+# define fallthrough                    do {} while (0)  /* fallthrough */
+#endif
+
 /*
  * Note the missing underscores.
  *