m68k: mm: Replace deprecated strncpy() with strscpy()

strncpy() is deprecated for NUL-terminated destination buffers. Use
strscpy() instead and remove the manual NUL-termination.

Compile-tested only.

Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Tested-by: Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>

arch/m68k/kernel/setup_mm.c

@@ -243,8 +243,7 @@ void __init setup_arch(char **cmdline_p)
 	setup_initial_init_mm((void *)PAGE_OFFSET, _etext, _edata, _end);
 
 #if defined(CONFIG_BOOTPARAM)
-	strncpy(m68k_command_line, CONFIG_BOOTPARAM_STRING, CL_SIZE);
-	m68k_command_line[CL_SIZE - 1] = 0;
+	strscpy(m68k_command_line, CONFIG_BOOTPARAM_STRING, CL_SIZE);
 #endif /* CONFIG_BOOTPARAM */
 	process_uboot_commandline(&m68k_command_line[0], CL_SIZE);
 	*cmdline_p = m68k_command_line;