btrfs: fix potential overflow in cluster_pages_for_defrag on 32bit arch

On 32-bit systems, this shift will overflow for files larger than 4GB as
start_index is unsigned long while the calls to btrfs_delalloc_*_space
expect u64.

CC: stable@vger.kernel.org # 4.4+
Fixes: df480633 ("btrfs: extent-tree: Switch to new delalloc space reserve and release")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
[ define the variable instead of repeating the shift ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -1274,6 +1274,7 @@ static int cluster_pages_for_defrag(struct inode *inode,
 	u64 page_start;
 	u64 page_end;
 	u64 page_cnt;
+	u64 start = (u64)start_index << PAGE_SHIFT;
 	int ret;
 	int i;
 	int i_done;
@@ -1290,8 +1291,7 @@ static int cluster_pages_for_defrag(struct inode *inode,
 	page_cnt = min_t(u64, (u64)num_pages, (u64)file_end - start_index + 1);
 
 	ret = btrfs_delalloc_reserve_space(BTRFS_I(inode), &data_reserved,
-			start_index << PAGE_SHIFT,
-			page_cnt << PAGE_SHIFT);
+			start, page_cnt << PAGE_SHIFT);
 	if (ret)
 		return ret;
 	i_done = 0;
@@ -1380,8 +1380,7 @@ static int cluster_pages_for_defrag(struct inode *inode,
 		btrfs_mod_outstanding_extents(BTRFS_I(inode), 1);
 		spin_unlock(&BTRFS_I(inode)->lock);
 		btrfs_delalloc_release_space(BTRFS_I(inode), data_reserved,
-				start_index << PAGE_SHIFT,
-				(page_cnt - i_done) << PAGE_SHIFT, true);
+				start, (page_cnt - i_done) << PAGE_SHIFT, true);
 	}
 
 
@@ -1408,8 +1407,7 @@ static int cluster_pages_for_defrag(struct inode *inode,
 		put_page(pages[i]);
 	}
 	btrfs_delalloc_release_space(BTRFS_I(inode), data_reserved,
-			start_index << PAGE_SHIFT,
-			page_cnt << PAGE_SHIFT, true);
+			start, page_cnt << PAGE_SHIFT, true);
 	btrfs_delalloc_release_extents(BTRFS_I(inode), page_cnt << PAGE_SHIFT);
 	extent_changeset_free(data_reserved);
 	return ret;