selftests: ir: fix warning: "%s" directive output may be truncated ’ directive output may be truncated

Fix the following warning by sizing the buffer to max. of sysfs
path max. size + d_name max. size.

gcc -Wall -O2 -I../../../include/uapi ir_loopback.c  -o ../tools/testing/selftests/ir/ir_loopback
ir_loopback.c: In function ‘lirc_open’:
ir_loopback.c:71:37: warning: ‘%s’ directive output may be truncated writing up to 255 bytes into a region of size 95 [-Wformat-truncation=]
    snprintf(buf, sizeof(buf), "/dev/%s", dent->d_name);
                                     ^~
In file included from /usr/include/stdio.h:862:0,
                 from ir_loopback.c:14:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:64:10: note: ‘__builtin___snprintf_chk’ output between 6 and 261 bytes into a destination of size 100
   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        __bos (__s), __fmt, __va_arg_pack ());
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Shuah Khan <shuah@kernel.org>
Acked-by: Sean Young <sean@mess.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>

tools/testing/selftests/ir/ir_loopback.c

@@ -27,6 +27,8 @@
 
 #define TEST_SCANCODES	10
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
+#define SYSFS_PATH_MAX 256
+#define DNAME_PATH_MAX 256
 
 static const struct {
 	enum rc_proto proto;
@@ -56,7 +58,7 @@ static const struct {
 int lirc_open(const char *rc)
 {
 	struct dirent *dent;
-	char buf[100];
+	char buf[SYSFS_PATH_MAX + DNAME_PATH_MAX];
 	DIR *d;
 	int fd;