Btrfs: clear pages dirty for io and set them extent mapped

When doing the io_ctl helpers to clean up the free space cache stuff I stopped
using our normal prepare_pages stuff, which means I of course forgot to do
things like set the pages extent mapped, which will cause us all sorts of
wonderful propblems.  Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/free-space-cache.c

@@ -351,6 +351,11 @@ static int io_ctl_prepare_pages(struct io_ctl *io_ctl, struct inode *inode,
 		}
 	}
 
+	for (i = 0; i < io_ctl->num_pages; i++) {
+		clear_page_dirty_for_io(io_ctl->pages[i]);
+		set_page_extent_mapped(io_ctl->pages[i]);
+	}
+
 	return 0;
 }