f2fs: don't allow negative ->write_io_size_bits

As Dan reported:

"We put an upper bound on ->write_io_size_bits but we don't have a lower
bound."

So let's add lower bound check for ->write_io_size_bits in parse_options().

[We don't allow configuring ->write_io_size_bits to zero, since at least
we need to fill one dummy page for aligned IO.]

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>