[CIFS] cifs truncate missing a fix for private map COW race

vmtruncate had added the same fix to handle the case of private pages
being Copy on writed while truncate_inode_pages is going on

Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/inode.c

diff --git a/fs/cifs/inode.c b/fs/cifs/inode.c
index dd41677..97ccc51 100644
--- a/fs/cifs/inode.c
+++ b/fs/cifs/inode.c
@@ -1377,8 +1377,17 @@
 	}
 	i_size_write(inode, offset);
 	spin_unlock(&inode->i_lock);
+	/*
+	 * unmap_mapping_range is called twice, first simply for efficiency
+	 * so that truncate_inode_pages does fewer single-page unmaps. However
+	 * after this first call, and before truncate_inode_pages finishes,
+	 * it is possible for private pages to be COWed, which remain after
+	 * truncate_inode_pages finishes, hence the second unmap_mapping_range
+	 * call must be made for correctness.
+	 */
 	unmap_mapping_range(mapping, offset + PAGE_SIZE - 1, 0, 1);
 	truncate_inode_pages(mapping, offset);
+	unmap_mapping_range(mapping, offset + PAGE_SIZE - 1, 0, 1);
 	goto out_truncate;
 
 do_expand: