bpf, sockmap: Use truesize with sk_rmem_schedule()

We use skb->size with sk_rmem_scheduled() which is not correct. Instead
use truesize to align with socket and tcp stack usage of sk_rmem_schedule.

Suggested-by: Daniel Borkman <daniel@iogearbox.net>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/160556570616.73229.17003722112077507863.stgit@john-XPS-13-9370