af_unix: Rename unlinked_skb in manage_oob().

When OOB skb has been already consumed, manage_oob() returns the next
skb if exists.  In such a case, we need to fall back to the else branch
below.

Then, we need to keep two skbs and free them later with consume_skb()
and kfree_skb().

Let's rename unlinked_skb accordingly.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240905193240.17565-3-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>