hwmon: (ibmpowernv) Replace deprecated strncpy() with memcpy()

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

A suitable replacement is `memcpy` as we've already precisely calculated
the number of bytes to copy while `buf` has been explicitly
zero-initialized:
| char buf[8] = { 0 };

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: [e-mail]
Signed-off-by: Justin Stitt <[e-mail]>
Tested-by: Michael Ellerman <[e-mail]>
Acked-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

drivers/hwmon/ibmpowernv.c

···
 	if (copy_len >= sizeof(buf))
 		return -EINVAL;
 
-	strncpy(buf, hash_pos + 1, copy_len);
+	memcpy(buf, hash_pos + 1, copy_len);
 
 	err = kstrtou32(buf, 10, index);
 	if (err)