mm: Fix fixup_user_fault() for MMU=n

In commit 2efaca927f ("mm/futex: fix futex writes on archs with SW
tracking of dirty & young") we forgot about MMU=n.  This patch fixes
that.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: David Howells <dhowells@redhat.com>
Link: http://lkml.kernel.org/r/1311761831.24752.413.camel@twins
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -962,6 +962,8 @@ int invalidate_inode_page(struct page *page);
 #ifdef CONFIG_MMU
 extern int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 			unsigned long address, unsigned int flags);
+extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
+			    unsigned long address, unsigned int fault_flags);
 #else
 static inline int handle_mm_fault(struct mm_struct *mm,
 			struct vm_area_struct *vma, unsigned long address,
@@ -971,6 +973,14 @@ static inline int handle_mm_fault(struct mm_struct *mm,
 	BUG();
 	return VM_FAULT_SIGBUS;
 }
+static inline int fixup_user_fault(struct task_struct *tsk,
+		struct mm_struct *mm, unsigned long address,
+		unsigned int fault_flags)
+{
+	/* should never happen if there's no MMU */
+	BUG();
+	return -EFAULT;
+}
 #endif
 
 extern int make_pages_present(unsigned long addr, unsigned long end);
@@ -988,8 +998,6 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			struct page **pages);
 struct page *get_dump_page(unsigned long addr);
-extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
-			    unsigned long address, unsigned int fault_flags);
 
 extern int try_to_release_page(struct page * page, gfp_t gfp_mask);
 extern void do_invalidatepage(struct page *page, unsigned long offset);