mm/kmemleak: rely on rcu for task stack scanning

kmemleak_scan() currently relies on the big tasklist_lock hammer to
stabilize iterating through the tasklist.  Instead, this patch proposes
simply using rcu along with the rcu-safe for_each_process_thread flavor
(without changing scan semantics), which doesn't make use of
next_thread/p->thread_group and thus cannot race with exit.  Furthermore,
any races with fork() and not seeing the new child should be benign as
it's not running yet and can also be detected by the next scan.

Avoiding the tasklist_lock could prove beneficial for performance
considering the scan operation is done periodically.  I have seen
improvements of 30%-ish when doing similar replacements on very
pathological microbenchmarks (ie stressing get/setpriority(2)).

However my main motivation is that it's one less user of the global
lock, something that Linus has long time wanted to see gone eventually
(if ever) even if the traditional fairness issues has been dealt with
now with qrwlocks.  Of course this is a very long ways ahead.  This
patch also kills another user of the deprecated tsk->thread_group.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Qian Cai <cai@lca.pw>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lkml.kernel.org/r/20200820203902.11308-1-dave@stgolabs.net
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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