tracing: add notrace to linkage.h

notrace signals that a function should not be traced. Most of the
time this is used by tracers to annotate code that cannot be
traced - it's in a volatile state (such as in user vdso context
or NMI context) or it's in the tracer internals.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/linkage.h

@@ -3,6 +3,8 @@
 
 #include <asm/linkage.h>
 
+#define notrace __attribute__((no_instrument_function))
+
 #ifdef __cplusplus
 #define CPP_ASMLINKAGE extern "C"
 #else