f2fs: fix remount problem of option io_bits

Currently we show mount option "io_bits=%u" as "io_size=%uKB",
it will cause option parsing problem(unrecognized mount option)
in remount.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>