Merge branch 'Wait for busy refill_work when destroying bpf memory allocator'

Hou Tao says:

====================

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset aims to fix one problem of bpf memory allocator destruction
when there is PREEMPT_RT kernel or kernel with arch_irq_work_has_interrupt()
being false (e.g. 1-cpu arm32 host or mips). The root cause is that
there may be busy refill_work when the allocator is destroying and it
may incur oops or other problems as shown in patch #1. Patch #1 fixes
the problem by waiting for the completion of irq work during destroying
and patch #2 is just a clean-up patch based on patch #1. Please see
individual patches for more details.

Comments are always welcome.

Change Log:
v2:
  * patch 1: fix typos and add notes about the overhead of irq_work_sync()
  * patch 1 & 2: add Acked-by tags from sdf@google.com

v1: https://lore.kernel.org/bpf/20221019115539.983394-1-houtao@huaweicloud.com/T/#t
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>