[PATCH v2 1/2] arm64: implement FTRACE_WITH_REGS

Once gcc is enhanced to optionally generate NOPs at the beginning
of each function, like the concept proven in
https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01671.html
(sans the "fprintf (... pad_size);", which spoils the data structure
for kernel use), the generated pads can nicely be used to reroute
function calls for tracing/profiling, or live patching.

diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index ebecf9a..917065c 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c

@@ -39,6 +39,12 @@ static int ftrace_modify_code(unsigned long pc, u32 old, u32 new,
 		if (aarch64_insn_read((void *)pc, &replaced))
 			return -EFAULT;
 
+		/* If we already have what we'll finally want,
+		 * report success. This is needed on startup.
+		 */
+		if (replaced == new)
+			return 0;