selftests/mm: fix strncpy() length

GCC complains (with -O2) that the length is equal to the destination size,
which is indeed invalid.  Subtract 1 from the size of the array to leave
room for '\0'.

Link: https://lkml.kernel.org/r/20241209095019.1732120-4-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/write_to_hugetlbfs.c

@@ -89,7 +89,7 @@ int main(int argc, char **argv)
 			size = atoi(optarg);
 			break;
 		case 'p':
-			strncpy(path, optarg, sizeof(path));
+			strncpy(path, optarg, sizeof(path) - 1);
 			break;
 		case 'm':
 			if (atoi(optarg) >= MAX_METHOD) {