mxser: Annotate struct mxser_board with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct mxser_board.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922175245.work.196-kees@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/mxser.c

@@ -288,7 +288,7 @@ struct mxser_board {
  enum mxser_must_hwid must_hwid;
  speed_t max_baud;
 
- struct mxser_port ports[];
+ struct mxser_port ports[] __counted_by(nports);
 };
 
 static DECLARE_BITMAP(mxser_boards, MXSER_BOARDS);