s390: move on_thread_stack() to processor.h

As preparation for the stackleak feature move on_thread_stack() to
processor.h like x86.

Also make it __always_inline, and slightly optimize it by reading
current task's kernel stack pointer from lowcore.
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/entry-common.h

@@ -60,9 +60,4 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 
 #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
 
-static inline bool on_thread_stack(void)
-{
-	return !(((unsigned long)(current->stack) ^ current_stack_pointer) & ~(THREAD_SIZE - 1));
-}
-
 #endif

arch/s390/include/asm/processor.h

@@ -226,6 +226,13 @@ static __always_inline unsigned long __current_stack_pointer(void)
 	return sp;
 }
 
+static __always_inline bool on_thread_stack(void)
+{
+	unsigned long ksp = S390_lowcore.kernel_stack;
+
+	return !((ksp ^ current_stack_pointer) & ~(THREAD_SIZE - 1));
+}
+
 static __always_inline unsigned short stap(void)
 {
 	unsigned short cpu_address;