af_unix: Set drop reason in unix_stream_read_skb().

unix_stream_read_skb() is called when BPF SOCKMAP reads some data
from a socket in the map.

SOCKMAP does not support MSG_OOB, and reading OOB results in a drop.

Let's set drop reasons respectively.

  * SOCKET_CLOSE  : the socket in SOCKMAP was close()d
  * UNIX_SKIP_OOB : OOB was read from the socket in SOCKMAP

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250116053441.5758-7-kuniyu@amazon.com


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