MIPS: r4k,octeon,r2300: stack protector: change canary per task

For non-SMP, uses the new random canary value that is stored in the
task struct whenever a new task is forked.  Based on ARM version in
df0698be14 and subject to the same
limitations: the variable GCC expects, __stack_chk_guard, is global,
so this will not work on SMP.

Quoting Nicolas Pitre <nico@fluxnic.net>: "One way to overcome this
GCC limitation would be to locate the __stack_chk_guard variable into
a memory page of its own for each CPU, and then use TLB locking to
have each CPU see its own page at the same virtual address for each of
them."

Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5488/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/asm-offsets.c

@@ -82,6 +82,9 @@ void output_task_defines(void)
 	OFFSET(TASK_FLAGS, task_struct, flags);
 	OFFSET(TASK_MM, task_struct, mm);
 	OFFSET(TASK_PID, task_struct, pid);
+#if defined(CONFIG_CC_STACKPROTECTOR)
+	OFFSET(TASK_STACK_CANARY, task_struct, stack_canary);
+#endif
 	DEFINE(TASK_STRUCT_SIZE, sizeof(struct task_struct));
 	BLANK();
 }