percpu_counter: add percpu_counter_sum_all interface

The percpu_counter is used for scenarios where performance is more
important than the accuracy.  For percpu_counter users, who want more
accurate information in their slowpath, percpu_counter_sum is provided
which traverses all the online CPUs to accumulate the data.  The reason it
only needs to traverse online CPUs is because percpu_counter does
implement CPU offline callback which syncs the local data of the offlined
CPU.

However there is a small race window between the online CPUs traversal of
percpu_counter_sum and the CPU offline callback.  The offline callback has
to traverse all the percpu_counters on the system to flush the CPU local
data which can be a lot.  During that time, the CPU which is going offline
has already been published as offline to all the readers.  So, as the
offline callback is running, percpu_counter_sum can be called for one
counter which has some state on the CPU going offline.  Since
percpu_counter_sum only traverses online CPUs, it will skip that specific
CPU and the offline callback might not have flushed the state for that
specific percpu_counter on that offlined CPU.

Normally this is not an issue because percpu_counter users can deal with
some inaccuracy for small time window.  However a new user i.e.  mm_struct
on the cleanup path wants to check the exact state of the percpu_counter
through check_mm().  For such users, this patch introduces
percpu_counter_sum_all() which traverses all possible CPUs and it is used
in fork.c:check_mm() to avoid the potential race.

This issue is exposed by the later patch "mm: convert mm's rss stats into
percpu_counter".

Link: https://lkml.kernel.org/r/20221109012011.881058-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>