bpf: Use bpf_mem_free_rcu when bpf_obj_dropping non-refcounted nodes

The use of bpf_mem_free_rcu to free refcounted local kptrs was added
in commit 7e26cd12 ("bpf: Use bpf_mem_free_rcu when
bpf_obj_dropping refcounted nodes"). In the cover letter for the
series containing that patch [0] I commented:

    Perhaps it makes sense to move to mem_free_rcu for _all_
    non-owning refs in the future, not just refcounted. This might
    allow custom non-owning ref lifetime + invalidation logic to be
    entirely subsumed by MEM_RCU handling. IMO this needs a bit more
    thought and should be tackled outside of a fix series, so it's not
    attempted here.

It's time to start moving in the "non-owning refs have MEM_RCU
lifetime" direction. As mentioned in that comment, using
bpf_mem_free_rcu for all local kptrs - not just refcounted - is
necessarily the first step towards that goal. This patch does so.

After this patch the memory pointed to by all local kptrs will not be
reused until RCU grace period elapses. The verifier's understanding of
non-owning ref validity and the clobbering logic it uses to enforce
that understanding are not changed here, that'll happen gradually in
future work, including further patches in the series.

  [0]: https://lore.kernel.org/all/20230821193311.3290257-1-davemarchevsky@fb.com/Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20231107085639.3016113-4-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -1937,10 +1937,7 @@ void __bpf_obj_drop_impl(void *p, const struct btf_record *rec, bool percpu)
 		ma = &bpf_global_percpu_ma;
 	else
 		ma = &bpf_global_ma;
-	if (rec && rec->refcount_off >= 0)
-		bpf_mem_free_rcu(ma, p);
-	else
-		bpf_mem_free(ma, p);
+	bpf_mem_free_rcu(ma, p);
 }
 
 __bpf_kfunc void bpf_obj_drop_impl(void *p__alloc, void *meta__ign)