powerpc/ps3: refactor strncpy usage

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

`make_first_field()` should use similar implementation to `make_field()`
due to memcpy having more obvious behavior here. The end result yields
the same behavior as the previous `strncpy`-based implementation
including the NUL-padding.

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 <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Tested-by: Geoff Levand <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/20230816[e-mail]

arch/powerpc/platforms/ps3/repository.c

···
 
 static u64 make_first_field(const char *text, u64 index)
 {
-	u64 n;
+	u64 n = 0;
 
-	strncpy((char *)&n, text, 8);
+	memcpy((char *)&n, text, strnlen(text, sizeof(n)));
 	return PS3_VENDOR_ID_NONE + (n >> 32) + index;
 }