bpf: Rename few bpf_mem_alloc fields.

Rename:
-       struct rcu_head rcu;
-       struct llist_head free_by_rcu;
-       struct llist_head waiting_for_gp;
-       atomic_t call_rcu_in_progress;
+       struct llist_head free_by_rcu_ttrace;
+       struct llist_head waiting_for_gp_ttrace;
+       struct rcu_head rcu_ttrace;
+       atomic_t call_rcu_ttrace_in_progress;
...
-	static void do_call_rcu(struct bpf_mem_cache *c)
+	static void do_call_rcu_ttrace(struct bpf_mem_cache *c)

to better indicate intended use.

The 'tasks trace' is shortened to 'ttrace' to reduce verbosity.
No functional changes.

Later patches will add free_by_rcu/waiting_for_gp fields to be used with normal RCU.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/bpf/20230706033447.54696-2-alexei.starovoitov@gmail.com