bcachefs: Fix NULL pointer dereference in bch2_opt_to_text

This patch adds a bounds check to the bch2_opt_to_text function to prevent
NULL pointer dereferences when accessing the opt->choices array. This
ensures that the index used is within valid bounds before dereferencing.
The new version enhances the readability.

Reported-and-tested-by: syzbot+37186860aa7812b331d5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=37186860aa7812b331d5Signed-off-by: Mohammed Anees <pvmohammedanees2003@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/opts.c

@@ -427,7 +427,9 @@ void bch2_opt_to_text(struct printbuf *out,
 			prt_printf(out, "%lli", v);
 		break;
 	case BCH_OPT_STR:
-		if (flags & OPT_SHOW_FULL_LIST)
+		if (v < opt->min || v >= opt->max - 1)
+			prt_printf(out, "(invalid option %lli)", v);
+		else if (flags & OPT_SHOW_FULL_LIST)
 			prt_string_option(out, opt->choices, v);
 		else
 			prt_str(out, opt->choices[v]);