af_unix: Set gc_in_progress to true in unix_gc().

In the Linux kernel, the following vulnerability has been resolved: af_unix: Set gc_in_progress to true in unix_gc(). Igor Ushakov reported that unix_gc() could run with gc_in_progress being false if the work is scheduled while running: Thread 1 Thread 2 Thread 3 -------- -------- -------- unix_schedule_gc() unix_schedule_gc() `- if (!gc_in_progress) `- if (!gc_in_progress) |- gc_in_progress = true | `- queue_work() | unix_gc() <----------------/ | | |- gc_in_progress = true ... `- queue_work() | | `- gc_in_progress = false | | unix_gc() <---------------------------------------------' | ... /* gc_in_progress == false */ | `- gc_in_progress = false unix_peek_fpl() relies on gc_in_progress not to confuse GC by MSG_PEEK. Let's set gc_in_progress to true in unix_gc().