rpmsg: Replace deprecated strncpy with strscpy_pad

rpmsg: 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.

We expect chinfo.name to be NUL-terminated based on its use with format
strings:
|       dev_err(&ctrldev->dev, "failed to create %s channel\n", chinfo.name);

Since chinfo is not default initialized, we should NUL-pad the `name`
field so that the behavior is consistent with what strncpy() provides:
|       struct rpmsg_channel_info chinfo;

Considering the above, a suitable replacement is `strscpy_pad` due to
the fact that it guarantees both NUL-termination and NUL-padding on the
destination buffer.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20231020-strncpy-drivers-rpmsg-rpmsg_ns-c-v1-1-99b16b00c36c@google.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>