selftests/bpf: Fix rcu_read_lock test with new MEM_RCU semantics

Add MEM_RCU pointer null checking for related tests. Also
modified task_acquire test so it takes a rcu ptr 'ptr' where
'ptr = rcu_ptr->rcu_field'.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221203184607.478314-1-yhs@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>