lkdtm: Force do_nothing() out of line

LKDTM tests display that the run do_nothing() at a given
address, but in reality do_nothing() is inlined into the
caller.

Force it out of line so that it really runs text at the
displayed address.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a5dcf4d2088e6aca47ab3b4c6d5c0f7fa064e25a.1644928018.git.christophe.leroy@csgroup.eu

drivers/misc/lkdtm/perms.c

@@ -21,7 +21,7 @@
 /* This is non-const, so it will end up in the .data section. */
 static u8 data_area[EXEC_SIZE];
 
-/* This is cost, so it will end up in the .rodata section. */
+/* This is const, so it will end up in the .rodata section. */
 static const unsigned long rodata = 0xAA55AA55;
 
 /* This is marked __ro_after_init, so it should ultimately be .rodata. */
@@ -31,7 +31,7 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
  * This just returns to the caller. It is designed to be copied into
  * non-executable memory regions.
  */
-static void do_nothing(void)
+static noinline void do_nothing(void)
 {
 	return;
 }