mm: fix build warning for uninitialized value

do_wp_page() sets mmun_called if mmun_start and mmun_end were
initialized and, if so, may call mmu_notifier_invalidate_range_end()
with these values.  This doesn't prevent gcc from emitting a build
warning though:

  mm/memory.c: In function `do_wp_page':
  mm/memory.c:2530: warning: `mmun_start' may be used uninitialized in this function
  mm/memory.c:2531: warning: `mmun_end' may be used uninitialized in this function

It's much easier to initialize the variables to impossible values and do
a simple comparison to determine if they were initialized to remove the
bool entirely.
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2527,9 +2527,8 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	int ret = 0;
 	int page_mkwrite = 0;
 	struct page *dirty_page = NULL;
-	unsigned long mmun_start;	/* For mmu_notifiers */
-	unsigned long mmun_end;		/* For mmu_notifiers */
-	bool mmun_called = false;	/* For mmu_notifiers */
+	unsigned long mmun_start = 0;	/* For mmu_notifiers */
+	unsigned long mmun_end = 0;	/* For mmu_notifiers */
 
 	old_page = vm_normal_page(vma, address, orig_pte);
 	if (!old_page) {
@@ -2708,8 +2707,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		goto oom_free_new;
 
 	mmun_start  = address & PAGE_MASK;
-	mmun_end    = (address & PAGE_MASK) + PAGE_SIZE;
-	mmun_called = true;
+	mmun_end    = mmun_start + PAGE_SIZE;
 	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
 
 	/*
@@ -2778,7 +2776,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		page_cache_release(new_page);
 unlock:
 	pte_unmap_unlock(page_table, ptl);
-	if (mmun_called)
+	if (mmun_end > mmun_start)
 		mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
 	if (old_page) {
 		/*