bcachefs: Change OPT_STR max to be 1 less than the size of choices array

Change OPT_STR max value to be 1 less than the "ARRAY_SIZE" of "_choices"
array. As a result, remove -1 from (opt->max-1) in bch2_opt_to_text.

The "_choices" array is a null-terminated array, so computing the maximum
using "ARRAY_SIZE" without subtracting 1 yields an incorrect result. Since
bch2_opt_validate don't subtract 1, as bch2_opt_to_text does, values
bigger than the actual maximum would pass through option validation.

Reported-by: syzbot+bee87a0c3291c06aa8c6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bee87a0c3291c06aa8c6
Fixes: 63c4b25 ("bcachefs: Better superblock opt validation")
Suggested-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Piotr Zalewski <pZ010001011111@proton.me>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: JungerBoyo

fs/bcachefs/opts.c

@@ -226,7 +226,7 @@ const struct bch_option bch2_opt_table[] = {
 #define OPT_UINT(_min, _max)	.type = BCH_OPT_UINT,			\
 				.min = _min, .max = _max
 #define OPT_STR(_choices)	.type = BCH_OPT_STR,			\
-				.min = 0, .max = ARRAY_SIZE(_choices),	\
+				.min = 0, .max = ARRAY_SIZE(_choices) - 1, \
 				.choices = _choices
 #define OPT_STR_NOLIMIT(_choices)	.type = BCH_OPT_STR,		\
 				.min = 0, .max = U64_MAX,		\
@@ -428,7 +428,7 @@ void bch2_opt_to_text(struct printbuf *out,
 			prt_printf(out, "%lli", v);
 		break;
 	case BCH_OPT_STR:
-		if (v < opt->min || v >= opt->max - 1)
+		if (v < opt->min || v >= opt->max)
 			prt_printf(out, "(invalid option %lli)", v);
 		else if (flags & OPT_SHOW_FULL_LIST)
 			prt_string_option(out, opt->choices, v);