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

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Linux kernel 安全漏洞

Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于af_unix驱动在OOB支持中存在pid泄漏。

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