af_unix: fix struct pid leaks in OOB support

In the Linux kernel, the following vulnerability has been resolved:

af_unix: fix struct pid leaks in OOB support

syzbot reported struct pid leak [1].

Issue is that queue_oob() calls maybe_add_creds() which potentially

holds a reference on a pid.

But skb->destructor is not set (either directly or by calling

unix_scm_to_skb())

This means that subsequent kfree_skb() or consume_skb() would leak

this reference.

In this fix, I chose to fully support scm even for the OOB message.

[1]

BUG: memory leak

unreferenced object 0xffff8881053e7f80 (size 128):

comm "syz-executor242", pid 5066, jiffies 4294946079 (age 13.220s)

hex dump (first 32 bytes):

01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

backtrace:

[<ffffffff812ae26a>] alloc_pid+0x6a/0x560 kernel/pid.c:180

[<ffffffff812718df>] copy_process+0x169f/0x26c0 kernel/fork.c:2285

[<ffffffff81272b37>] kernel_clone+0xf7/0x610 kernel/fork.c:2684

[<ffffffff812730cc>] __do_sys_clone+0x7c/0xb0 kernel/fork.c:2825

[<ffffffff849ad699>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]

[<ffffffff849ad699>] do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80

[<ffffffff84a0008b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd