ARM: 9150/1: Fix PID_IN_CONTEXTIDR regression when THREAD_INFO_IN_TASK=y

The code that implements the rarely used PID_IN_CONTEXTIDR feature
dereferences the 'task' field of struct thread_info directly, and this
is no longer possible when THREAD_INFO_IN_TASK=y, as the 'task' field is
omitted from the struct definition in that case. Instead, we should just
cast the thread_info pointer to a task_struct pointer, given that the
former is now the first member of the latter.

So use a helper that abstracts this, and provide implementations for
both cases.

Reported by: Arnd Bergmann <arnd@arndb.de>

Fixes: 18ed1c01 ("ARM: smp: Enable THREAD_INFO_IN_TASK")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

arch/arm/include/asm/thread_info.h

@@ -79,9 +79,20 @@ struct thread_info {
 
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 #define INIT_THREAD_INFO_TASK(tsk)
+
+static inline struct task_struct *thread_task(struct thread_info* ti)
+{
+	return (struct task_struct *)ti;
+}
+
 #else
 #define INIT_THREAD_INFO_TASK(tsk)	.task = &(tsk),
 
+static inline struct task_struct *thread_task(struct thread_info* ti)
+{
+	return ti->task;
+}
+
 /*
  * how to get the thread information struct from C
  */

arch/arm/mm/context.c

@@ -109,7 +109,7 @@ static int contextidr_notifier(struct notifier_block *unused, unsigned long cmd,
 	if (cmd != THREAD_NOTIFY_SWITCH)
 		return NOTIFY_DONE;
 
-	pid = task_pid_nr(thread->task) << ASID_BITS;
+	pid = task_pid_nr(thread_task(thread)) << ASID_BITS;
 	asm volatile(
 	"	mrc	p15, 0, %0, c13, c0, 1\n"
 	"	and	%0, %0, %2\n"