isdn: kcapi: replace deprecated strncpy with strscpy_pad

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

`buf` is used in this context as a data buffer with 64 bytes of memory
to be occupied by capi_manufakturer.

We see the caller capi20_get_manufacturer() passes data.manufacturer as
its `buf` argument which is then later passed over to user space. Due to
this, let's keep the NUL-padding that strncpy provided by using
strscpy_pad so as to not leak any stack data.
| cdev->errcode = capi20_get_manufacturer(data.contr, data.manufacturer);
| if (cdev->errcode)
| return -EIO;
|
| if (copy_to_user(argp, data.manufacturer,
| sizeof(data.manufacturer)))
| return -EFAULT;

Perhaps this would also be a good instance to use `strtomem_pad` for but
in my testing the compiler was not able to determine the size of `buf`
-- even with all the hints.

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: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230922-strncpy-drivers-isdn-capi-kcapi-c-v1-1-55fcf8b075fb@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>