af_unix: Don't call skb_get() for OOB skb.

af_unix: Don't call skb_get() for OOB skb.

Since introduced, OOB skb holds an additional reference count with no
special reason and caused many issues.

Also, kfree_skb() and consume_skb() are used to decrement the count,
which is confusing.

Let's drop the unnecessary skb_get() in queue_oob() and corresponding
kfree_skb(), consume_skb(), and skb_unref().

Now unix_sk(sk)->oob_skb is just a pointer to skb in the receive queue,
so special handing is no longer needed in GC.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240816233921.57800-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>