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 via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

Add __counted_by for struct p54_cal_database.

Cc: Christian Lamparter <chunkeey@googlemail.com>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-wireless@vger.kernel.org
Cc: linux-hardening@vger.kernel.org
Suggested-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231009161028.it.544-kees@kernel.org