f2fs: flush pending checkpoints when freezing super

commit c7b58576 upstream.

This avoids -EINVAL when trying to freeze f2fs.

Cc: stable@vger.kernel.org
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>