[BUG] f2fs-tools: fsck infinite loop in options parsing

[Egor Shestakov <vedingrot@gmail.com>]

Hi!

I found a bug in the fsck.f2fs that cause infinite loop in
f2fs_parse_options().  To reproduce it you can call program with a
'-py' united options combination, but when it's separate '-p -y'
works well. Simply execute `./f2fs.fsck -py` to catch the bug.

Buggy part of a fsck/main.c code (unchanged since 2018):
>		case 'p':
>			/* preen mode has different levels:
>			 *  0: default level, the same as -a
>			 *  1: check meta
>			 *  2: same as 0, but will skip some
>			 *     check for old kernel
>			 */
>			if (optarg[0] == '-' || !is_digits(optarg) ||
>						optind == argc) {
>				MSG(0, "Info: Use default preen mode\n");
>				c.preen_mode = PREEN_MODE_0;
>				c.auto_fix = 1;
>				optind--;
>				break;
>			}

The bug occurs when a case 'p' match and after it there is not
suitable argument so a decrement optind-- happened. Since the
option '-p' united with its argument a getopt increments optind
only by one, not by two, as expected. Therefore it enters to
infinite loop.

I couldn't find good solution. Changing a preen level options
semantic is impossible because breaks many scripts, for example
in initrd.  Possible solution is use a two colons in optstring
that means optinal argument, but this is a GNU extension, so not
all standard libraries support it, in particular Musl.

-- 
Egor Shestakov
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