[PATCH 5.10.162-rt78] Restore initialization of wake_q_sleeper.next in fork.c

[Joe Korty <joe.korty@concurrent-rt.com>]

In the transition from 5.10.158-rt77 to 5.10.162-rt78,
the initialization of task_struct::wake_q_sleeper.next
was dropped.  Restore it.

This appears to be only a problem in 5.10.  5.15 does not
have wake_q_sleeper; 4.19 does have it but its initialization
there is still present.

The 5.10.162-rt78 patch that damaged fork.c is:

   0170-locking-rtmutex-add-sleeping-lock-implementation.patch

I do not have a simple test that brings out this problem.
My test consists of a shell script and eight binaries,
all of which were written in Ada.  strace shows that it
does a few thousand forks in rapid succession.  One of the
forks stalls out, after which no fork after that returns.
Eventually the 122 second stallout occurs and a large
number of threads are shown to be waiting for tasklist
lock, either in do_exit or in copy_process.  The kernel
.config has rt and many debug features enabled, lockdep
included.

Signed-off-by: Joe Korty <joe.korty@concurrent-rt.com

Index: b/kernel/fork.c
===================================================================
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -960,6 +960,7 @@ static struct task_struct *dup_task_stru
 	tsk->splice_pipe = NULL;
 	tsk->task_frag.page = NULL;
 	tsk->wake_q.next = NULL;
+	tsk->wake_q_sleeper.next = NULL;
 	tsk->pf_io_worker = NULL;
 
 	account_kernel_stack(tsk, 1);