ACPI: OSI: refactor deprecated strncpy()

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

We know `osi->string` is a NUL-terminated string due to its eventual use
in `acpi_install_interface()` and `acpi_remove_interface()` which expect
a `acpi_string` which has been specifically typedef'd as:
|  typedef char *acpi_string;	/* Null terminated ASCII string */

... and which also has other string functions used on it like `strlen`.
Furthermore, padding is not needed in this instance either.

Due to the reasoning above a suitable replacement is `strscpy` [2] since
it guarantees NUL-termination on the destination buffer and doesn't
unnecessarily NUL-pad.

While there is unlikely to be a buffer overread (or other related bug)
in this case, we should still favor a more robust and less ambiguous
interface.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: KSPP#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>

Author: JustinStitt

drivers/acpi/osi.c

@@ -110,7 +110,7 @@ void __init acpi_osi_setup(char *str)
 			break;
 		} else if (osi->string[0] == '\0') {
 			osi->enable = enable;
-			strncpy(osi->string, str, OSI_STRING_LENGTH_MAX);
+			strscpy(osi->string, str, OSI_STRING_LENGTH_MAX);
 			break;
 		}
 	}