If backlog handler is running during a tear down operation we may enqueue
data on the ingress msg queue while tear down is trying to free it.

 sk_psock_backlog()
   sk_psock_handle_skb()
     skb_psock_skb_ingress()
       sk_psock_skb_ingress_enqueue()
         sk_psock_queue_msg(psock,msg)
                                           spin_lock(ingress_lock)
                                            sk_psock_zap_ingress()
                                             _sk_psock_purge_ingerss_msg()
                                              _sk_psock_purge_ingress_msg()
                                            -- free ingress_msg list --
                                           spin_unlock(ingress_lock)
           spin_lock(ingress_lock)
           list_add_tail(msg,ingress_msg) <- entry on list with no one
                                             left to free it.
           spin_unlock(ingress_lock)

To fix we only enqueue from backlog if the ENABLED bit is set. The tear
down logic clears the bit with ingress_lock set so we wont enqueue the
msg in the last step.

Fixes: 799aa7f9 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210727160500.1713554-4-john.fastabend@gmail.com