RPi: add proposed upstream linux patch to fix ftrace on boot

see http://lists.infradead.org/pipermail/linux-rpi-kernel/2017-August/006884.html

Signed-off-by: Matthias Reichl <hias@horus.com>

projects/RPi/patches/linux/linux-06-fix-ftrace-on-boot.patch

@@ -0,0 +1,57 @@
+On Wed, 23 Aug 2017 17:42:14 +0200
+Matthias Reichl <hias@horus.com> wrote:
+
+> I haven't verified it myself but Phil Elwells first analysis here
+> sounds plausible
+> 
+> https://github.com/raspberrypi/linux/issues/2166#issuecomment-323355145
+
+I noticed this comment:
+
+"What makes this interesting (and is the reason this doesn't fail
+ normally) is that these functions are all called within kernel_init, so
+ the __ro_after_init attribute shouldn't have caused the variable to be
+ read-only yet."
+
+When ftrace starts, it needs to set the kernel text to write, and when
+it finishes it sets it back to readonly. But the kernel is not set to
+readonly at boot up.
+
+Can you test this patch? (Warning, I didn't even compile test it)
+
+-- Steve
+
+diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
+index ad80548..fd75f38 100644
+--- a/arch/arm/mm/init.c
++++ b/arch/arm/mm/init.c
+@@ -745,19 +745,29 @@ static int __mark_rodata_ro(void *unused)
+ 	return 0;
+ }
+ 
++static int kernel_set_to_readonly;
++
+ void mark_rodata_ro(void)
+ {
++	kernel_set_to_readonly = 1;
++
+ 	stop_machine(__mark_rodata_ro, NULL, NULL);
+ }
+ 
+ void set_kernel_text_rw(void)
+ {
++	if (!kernel_set_to_readonly)
++		return;
++
+ 	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,
+ 				current->active_mm);
+ }
+ 
+ void set_kernel_text_ro(void)
+ {
++	if (!kernel_set_to_readonly)
++		return;
++
+ 	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
+ 				current->active_mm);
+ }