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 <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Shawn Guo <[email protected]>
Cc: Sascha Hauer <[email protected]>
Cc: Pengutronix Kernel Team <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: NXP Linux Team <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Jiri Slaby <[email protected]>
Reviewed-by: "Gustavo A. R. Silva" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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);