Fix capabilities plugin on kernels before 6.3

[mmadersbacher]

linux.capabilities aborts on any image whose symbols carry kernel_cap_t in the pre-6.3 layout:

VolatilityException: Unsupported kernel capabilities implementation

kernel_cap_t was added for the 6.3 rework (#985), where the struct became { u64 val; }. Before 6.3 the same name exists as a typedef of kernel_cap_struct with a two-element __u32 cap[], and symbols/linux/__init__.py:61 maps that name to the subclass regardless. The subclass only handles val, so it raises even though kernel_cap_struct.get_capabilities() decodes the old layout correctly.

Falling back to the parent keeps the error path intact. It still raises the same exception when cap is missing or has an unexpected length (extensions/__init__.py:2310 and :2328).

All three Debian ISFs used by the test workflow carry the old layout:

kernel_cap_t  size=8  fields: ['cap: array[2] of unsigned int']

On linux-sample-1.bin the exit code goes from 1 to 0 and the plugin lists 133 tasks instead of none. test_linux_generic_capabilities no longer takes its early-return path for this image, so its assertions actually run now.

ruff check and ruff format --check clean. test/plugins/linux/linux.py 41 passed.

[ikelos]

Looks reasonable, if the tests pass we'll merge it, thanks! 5:)