eeprom: idt_89hpesx: replace open-coded kmemdup_nul

A malloc + strncpy + manual NUL_termination is just kmemdup_nul. Let's use
this interface as it is less error-prone and more readable.

Also drop `csraddr_len` as it is just used in a single place and we can
just do the arithmetic in-line.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: [e-mail]
Cc: Kees Cook <[e-mail]>
Signed-off-by: Justin Stitt <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/2023[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/eeprom/idt_89hpesx.c

···
 {
 	struct idt_89hpesx_dev *pdev = filep->private_data;
 	char *colon_ch, *csraddr_str, *csrval_str;
-	int ret, csraddr_len;
+	int ret;
 	u32 csraddr, csrval;
 	char *buf;
 
···
 	 * no new CSR value
 	 */
 	if (colon_ch != NULL) {
-		csraddr_len = colon_ch - buf;
-		csraddr_str =
-			kmalloc(csraddr_len + 1, GFP_KERNEL);
+		/* Copy the register address to the substring buffer */
+		csraddr_str = kmemdup_nul(buf, colon_ch - buf, GFP_KERNEL);
 		if (csraddr_str == NULL) {
 			ret = -ENOMEM;
 			goto free_buf;
 		}
-		/* Copy the register address to the substring buffer */
-		strncpy(csraddr_str, buf, csraddr_len);
-		csraddr_str[csraddr_len] = '\0';
 		/* Register value must follow the colon */
 		csrval_str = colon_ch + 1;
 	} else /* if (str_colon == NULL) */ {
 		csraddr_str = (char *)buf; /* Just to shut warning up */
-		csraddr_len = strnlen(csraddr_str, count);
 		csrval_str = NULL;
 	}