f2fs: fix checkpoint=disable:%u%%

When parsing the mount option, we don't have sbi->user_block_count.
Should do it after getting it.

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