PNP: replace deprecated strncpy() with memcpy()

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

After having precisely calculated the lengths and ensuring we don't
overflow the buffer, this really decays to just a memcpy. Let's not use
a C string api as it makes the intention of the code confusing.

It'd be nice to use strscpy() in this case (as we clearly want
NUL-termination) because it'd clean up the code a bit. However, I don't
quite know enough about what is going on here to justify a drop-in
replacement -- too much bit magic and why (PNP_NAME_LEN - 2)? I'm afraid
using strscpy() may result in copying too many or too few bytes into our
dev->name buffer resulting in different behavior. At least using
memcpy() we can ensure the behavior is exactly the same.

Side note:
NUL-padding is not required because insert_device() calls
pnpbios_parse_data_stream() with a zero-allocated `dev`:
299 |  static int __init insert_device(struct pnp_bios_node *node) {
...
312 |  dev = pnp_alloc_dev(&pnpbios_protocol, node->handle, id);
...
316 |  pnpbios_parse_data_stream(dev, node);

then pnpbios_parse_data_stream() calls pnpbios_parse_compatible_ids().

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


Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>