This patch adds test cases for iter next method returning valid
pointer, which can also used as usage examples.

Currently iter next method should return valid pointer.

iter_next_trusted is the correct usage and test if iter next method
return valid pointer. bpf_iter_task_vma_next has KF_RET_NULL flag,
so the returned pointer may be NULL. We need to check if the pointer
is NULL before using it.

iter_next_trusted_or_null is the incorrect usage. There is no checking
before using the pointer, so it will be rejected by the verifier.

iter_next_rcu and iter_next_rcu_or_null are similar test cases for
KF_RCU_PROTECTED iterators.

iter_next_rcu_not_trusted is used to test that the pointer returned by
iter next method of KF_RCU_PROTECTED iterator cannot be passed in
KF_TRUSTED_ARGS kfuncs.

iter_next_ptr_mem_not_trusted is used to test that base type
PTR_TO_MEM should not be combined with type flag PTR_TRUSTED.
Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Link: https://lore.kernel.org/r/AM6PR03MB5848709758F6922F02AF9F1F99962@AM6PR03MB5848.eurprd03.prod.outlook.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>