thunderbolt: tunnel: Simplify allocation

Use a flexible array member and kzalloc_flex to combine allocations.

Add __counted_by for extra runtime analysis. Move counting variable
assignment after allocation. kzalloc_flex with GCC >= 15 does this
automatically.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>