selftests/bpf: update verifier test for default trusted pointer semantics

selftests/bpf: update verifier test for default trusted pointer semantics

Replace the verifier test for default trusted pointer semantics, which
previously relied on BPF kfunc bpf_get_root_mem_cgroup(), with a new
test utilizing dedicated BPF kfuncs defined within the bpf_testmod.

bpf_get_root_mem_cgroup() was modified such that it again relies on
KF_ACQUIRE semantics, therefore no longer making it a suitable
candidate to test BPF verifier default trusted pointer semantics
against.

Link: https://lore.kernel.org/bpf/20260113083949.2502978-2-mattbobrowski@google.com
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Link: https://lore.kernel.org/r/20260120091630.3420452-1-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>