selftests/bpf: Test task storage when local_storage->smap is NULL

The current sk storage test ensures the memory free works when
the local_storage->smap is NULL.

This patch adds a task storage test to ensure the memory free
code path works when local_storage->smap is NULL.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20230322215246.1675516-5-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>