f2fs: check free sections before disable checkpoint

'f2fs_is_checkpoint_ready()' checks free sections. If there is not
enough free sections, most f2fs operations will return -ENOSPC when
checkpoint is disabled.

It would be better to check free sections before disable checkpoint.

Signed-off-by: Wu Bo <bo.wu@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>