selftests/bpf: Test __nullable pointer members

Add a struct_ops member to bpf_testmod whose argument struct carries two
task pointers, one plain and one __nullable, both made trusted through a
__safe_trusted alias of the struct in the module BTF, and a kfunc that
calls the member once with the nullable pointer unset and once set.

The callback acquires the plain pointer as is and the nullable one only
after a check, driven through the kfunc from a syscall program. The
failure case passes the nullable pointer to bpf_task_acquire() unchecked
and is rejected as possibly NULL.

More members cover the other kinds of pointer: a per-CPU one, which
only the per-CPU helpers consume and which keeps the flag, the struct
itself through a plain pointer and through an untrusted one out of a
__kptr_untrusted map value that is never set, and a pointer to a scalar,
which are probed and lose it. The callback uses each after a check; a
failure test passes the per-CPU pointer unchecked, and three programs
read the probed ones unchecked and load. bpf_testmod_ops3 gets the base
helpers for the per-CPU helpers.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261019132203.3034698-9-memxor@gmail.com

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