f2fs: fix to spread f2fs_is_checkpoint_ready()

We missed to call f2fs_is_checkpoint_ready() in several places, it may
allow space allocation even when free space was exhausted during
checkpoint is disabled, fix to add them.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>