bpf: Let bpf_iter_task_new accept null task ptr

When using task_iter to iterate all threads of a specific task, we enforce
that the user must pass a valid task pointer to ensure safety. However,
when iterating all threads/process in the system, BPF verifier still
require a valid ptr instead of "nullable" pointer, even though it's
pointless, which is a kind of surprising from usability standpoint. It
would be nice if we could let that kfunc accept a explicit null pointer
when we are using BPF_TASK_ITER_ALL_{PROCS, THREADS} and a valid pointer
when using BPF_TASK_ITER_THREAD.

Given a trival kfunc:
	__bpf_kfunc void FN(struct TYPE_A *obj);

BPF Prog would reject a nullptr for obj. The error info is:
"arg#x pointer type xx xx must point to scalar, or struct with scalar"
reported by get_kfunc_ptr_arg_type(). The reg->type is SCALAR_VALUE and
the btf type of ref_t is not scalar or scalar_struct which leads to the
rejection of get_kfunc_ptr_arg_type.

This patch add "__nullable" annotation:
	__bpf_kfunc void FN(struct TYPE_A *obj__nullable);
Here __nullable indicates obj can be optional, user can pass a explicit
nullptr or a normal TYPE_A pointer. In get_kfunc_ptr_arg_type(), we will
detect whether the current arg is optional and register is null, If so,
return a new kfunc_ptr_arg_type KF_ARG_PTR_TO_NULL and skip to the next
arg in check_kfunc_args().

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231018061746.111364-7-zhouchuyi@bytedance.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>