Btrfs: btrfs_release_extent_buffer_page didn't free pages of dummy extent

btrfs_release_extent_buffer_page() can't handle dummy extent that
allocated by btrfs_clone_extent_buffer() properly. That is because
reference count of pages that allocated by btrfs_clone_extent_buffer()
was 2, 1 by alloc_page(), and another by attach_extent_buffer_page().

Running following command repeatly can check this memory leak problem

    btrfs inspect-internal inode-resolve 256 /mnt/btrfs
Signed-off-by: NChien-Kuan Yeh <ckya@synology.com>
Signed-off-by: NForrest Liu <forrestl@synology.com>
Reviewed-by: NFilipe Manana <fdmanana@suse.com>
Tested-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/extent_io.c

@@ -4560,7 +4560,9 @@ static void btrfs_release_extent_buffer_page(struct extent_buffer *eb)
 	do {
 		index--;
 		page = eb->pages[index];
-		if (page && mapped) {
+		if (!page)
+			continue;
+		if (mapped)
 			spin_lock(&page->mapping->private_lock);
 		/*
 		 * We do this since we'll remove the pages after we've
@@ -4583,13 +4585,12 @@ static void btrfs_release_extent_buffer_page(struct extent_buffer *eb)
 			/* One for the page private */
 			page_cache_release(page);
 		}
+
+		if (mapped)
 			spin_unlock(&page->mapping->private_lock);
 
-		}
-		if (page) {
 		/* One for when we alloced the page */
 		page_cache_release(page);
-		}
 	} while (index != 0);
 }