rpmsg: core: Replace deprecated strncpy with strscpy

rpmsg: core: Replace deprecated strncpy with strscpy

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 usage with
strncmp():

rpmsg_core.c:
389: if (strncmp(chinfo->name, rpdev->id.name, RPMSG_NAME_SIZE))

Moreover, NUL-padding is not required as chinfo has stack default
initialized all fields to zero:

rpmsg_core.c:
539: struct rpmsg_channel_info chinfo = {};

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

Also, favor the more idiomatic strscpy() usage of:
(dest, src, sizeof(dest)).

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html
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_core-c-v1-1-a86b7930c1cf@google.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>