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/[e-mail]
Signed-off-by: Kevin Brodsky <[e-mail]>
Cc: Aruna Ramakrishna <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Joey Gouly <[e-mail]>
Cc: Keith Lucas <[e-mail]>
Cc: Ryan Roberts <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/write_to_hugetlbfs.c

···
 			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) {