selftests/bpf: Test small task local data allocation

selftests/bpf: Test small task local data allocation

Make sure task local data is working correctly for different allocation
sizes. Existing task local data selftests allocate the maximum amount of
data possible but miss the garbage data issue when only small amount of
data is allocated. Therefore, test small data allocations as well.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260413190259.358442-4-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>