ASoC: sigmadsp: Add __counted_by for struct sigmadsp_data and use struct_size()

ASoC: sigmadsp: Add __counted_by for struct sigmadsp_data and use struct_size()

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).

While there, use struct_size() and size_sub() helpers, instead of the
open-coded version, to calculate the size for the allocation of the
whole flexible structure, including of course, the flexible-array
member.

This code was found with the help of Coccinelle, and audited and
fixed manually.

Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/ZSRvh1j2MVVhuOUv@work
Signed-off-by: Mark Brown <broonie@kernel.org>