mkfs.f2fs: not allow to format too small sized partition

mkfs.f2fs: not allow to format too small sized partition

For example, if we want to format 40MB image, it'll give 12 segments for main
area. And, it assigns 6 for current segments, 11 for ovp segments, and 10 for
reserved segments. So, we have 5 free segments, which we can't handle:
 # of free segments < # of reserved segments.

Reported-by: Daniel Rosenberg <drosen@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>