btrfs: Refactor loop in btrfs_release_extent_buffer_page

The purpose of the function is to free all the pages comprising an
extent buffer. This can be achieved with a simple for loop rather than
the slightly more involved 'do {} while' construct. So rewrite the
loop using a 'for' construct. Additionally we can never have an
extent_buffer that has 0 pages so remove the check for index == 0. No
functional changes.

The reversed order used to have a meaning in the past where the first
page served as a blocking point for several callers. See eg
4f2de97acee6532b36dd6e99 ("Btrfs: set page->private to the eb").

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -4647,19 +4647,16 @@ int extent_buffer_under_io(struct extent_buffer *eb)
  */
 static void btrfs_release_extent_buffer_page(struct extent_buffer *eb)
 {
-	int index;
-	struct page *page;
+	int i;
+	int num_pages;
 	int mapped = !test_bit(EXTENT_BUFFER_DUMMY, &eb->bflags);
 
 	BUG_ON(extent_buffer_under_io(eb));
 
-	index = num_extent_pages(eb);
-	if (index == 0)
-		return;
+	num_pages = num_extent_pages(eb);
+	for (i = 0; i < num_pages; i++) {
+		struct page *page = eb->pages[i];
 
-	do {
-		index--;
-		page = eb->pages[index];
 		if (!page)
 			continue;
 		if (mapped)
@@ -4691,7 +4688,7 @@ static void btrfs_release_extent_buffer_page(struct extent_buffer *eb)
 
 		/* One for when we allocated the page */
 		put_page(page);
-	} while (index != 0);
+	}
 }
 
 /*