arm64: Add regs_return_value() in syscall.h

This macro, regs_return_value, is used mainly for audit to record system
call's results, but may also be used in test_kprobes.c.
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/ptrace.h

@@ -135,6 +135,11 @@ struct pt_regs {
 #define user_stack_pointer(regs) \
 	(!compat_user_mode(regs)) ? ((regs)->sp) : ((regs)->compat_sp)
 
+static inline unsigned long regs_return_value(struct pt_regs *regs)
+{
+	return regs->regs[0];
+}
+
 /*
  * Are the current registers suitable for user mode? (used to maintain
  * security in signal handlers)