selftests/bpf: Add test to verify freeing the special fields in pcpu maps

selftests/bpf: Add test to verify freeing the special fields in pcpu maps

Add test to verify that updating [lru_,]percpu_hash maps decrements
refcount when BPF_KPTR_REF objects are involved.

The tests perform the following steps:
. Call update_elem() to insert an initial value.
. Use bpf_refcount_acquire() to increment the refcount.
. Store the node pointer in the map value.
. Add the node to a linked list.
. Probe-read the refcount and verify it is *2*.
. Call update_elem() again to trigger refcount decrement.
. Probe-read the refcount and verify it is *1*.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20251105151407.12723-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>