[POWERPC] Add thread_info_cache_init() weak hook

Some architectures need to maintain a kmem cache for thread info
structures.  The next commit adds that to powerpc to fix an alignment
problem.

There is no good arch callback to use to initialize that cache
that I can find, so this adds a new one in the form of a weak
function whose default is empty.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/linux/sched.h

@@ -1926,6 +1926,8 @@ static inline unsigned long *end_of_stack(struct task_struct *p)
 
 #endif
 
+extern void thread_info_cache_init(void);
+
 /* set thread flags in other task's structures
  * - see asm/thread_info.h for TIF_xxxx flags available
  */

init/main.c

@@ -525,6 +525,10 @@ void __init __attribute__((weak)) smp_setup_processor_id(void)
 {
 }
 
+void __init __weak thread_info_cache_init(void)
+{
+}
+
 asmlinkage void __init start_kernel(void)
 {
 	char * command_line;
@@ -645,6 +649,7 @@ asmlinkage void __init start_kernel(void)
 	if (efi_enabled)
 		efi_enter_virtual_mode();
 #endif
+	thread_info_cache_init();
 	fork_init(num_physpages);
 	proc_caches_init();
 	buffer_init();