bpf: allow using bpf_kptr_xchg even if the MEM_RCU flag is set

bpf: allow using bpf_kptr_xchg even if the MEM_RCU flag is set

For the following scenario:
    struct tree_node {
	struct bpf_refcount ref;
	struct bpf_rb_node node;
	struct node_data __kptr * node_data;
	u64 key;
    };
This means node_data would have the type PTR_TO_BTF_ID | MEM_ALLOC |
NON_OWN_REF | MEM_RCU.

When traversing an rbtree using bpf_rbtree_left/right, if we need to
use bpf_kptr_xchg to read the __kptr pointer, we still need to follow
the remove-read-add sequence.

This patch allows us to use bpf_kptr_xchg to directly read the __kptr
pointer without any prior operations.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Link: https://lore.kernel.org/r/20260214124042.62229-5-pilgrimtao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>