net: Fix rcu_tasks stall in threaded busypoll

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

net: Fix rcu_tasks stall in threaded busypoll

I was debugging a NIC driver when I noticed that when I enable
threaded busypoll, bpftrace hangs when starting up. dmesg showed:

rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 10658 jiffies old.
rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 40793 jiffies old.
rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 131273 jiffies old.
rcu_tasks_wait_gp: rcu_tasks grace period number 85 (since boot) is 402058 jiffies old.
INFO: rcu_tasks detected stalls on tasks:
00000000769f52cd: .N nvcsw: 2/2 holdout: 1 idle_cpu: -1/64
task:napi/eth2-8265 state:R running task stack:0 pid:48300 tgid:48300 ppid:2 task_flags:0x208040 flags:0x00004000
Call Trace:
<TASK>
? napi_threaded_poll_loop+0x27c/0x2c0
? __pfx_napi_threaded_poll+0x10/0x10
? napi_threaded_poll+0x26/0x80
? kthread+0xfa/0x240
? __pfx_kthread+0x10/0x10
? ret_from_fork+0x31/0x50
? __pfx_kthread+0x10/0x10
? ret_from_fork_asm+0x1a/0x30
</TASK>

The cause is that in threaded busypoll, the main loop is in
napi_threaded_poll rather than napi_threaded_poll_loop, where the
latter rarely iterates more than once within its loop. For
rcu_softirq_qs_periodic inside napi_threaded_poll_loop to report its
qs state, the last_qs must be 100ms behind, and this can't happen
because napi_threaded_poll_loop rarely iterates in threaded busypoll,
and each time napi_threaded_poll_loop is called last_qs is reset to
latest jiffies.

This patch changes so that in threaded busypoll, last_qs is saved
in the outer napi_threaded_poll, and whether busy_poll_last_qs
is NULL indicates whether napi_threaded_poll_loop is called for
busypoll. This way last_qs would not reset to latest jiffies on
each invocation of napi_threaded_poll_loop.