hsi: hsi_core: use kzalloc_flex

Simplifies allocations by using a flexible array member in this struct.

Add __counted_by to get extra runtime analysis.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260318191037.5661-1-rosenp@gmail.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>