Kmemleak infrastructure improvement for task_struct leaks and call_rcu()

[Qian Cai <cai@lca.pw>]

== task struck leaks ==
There are leaks from task struct from time to time where someone forgot to call put_task_struct() somewhere leading to leaks. For example,

https://lore.kernel.org/lkml/C1CCBDAC-A453-4FF2-908F-0B6E356223D1@lca.pw/

It was such a pain to debug this kind of leaks at the moment, as all we could do was to audit the code by checking all new put_task_struct()  and get_task_struct() call sites which is error-prone because there could be other new call sites like get_pid_task() which would also need to be balanced with put_task_struct() as well.

What do you think about adding some aux call traces for kmemleak in general? For example, if the tracking object is a task struct, it would save call traces for the first and last call of both get_task_struct() and put_task_struct(). Then, it could be expanded to track other refcount-based leaks in the future.

== call_rcu() leaks ==
Another issue that might be relevant is that it seems sometimes, kmemleak will give a lot of false positives (hundreds) because the memory was supposed to be freed by call_rcu()  (for example, in dst_release()) but for some reasons, it takes a long time probably waiting for grace periods or some kind of RCU self-stall, but the memory had already became an orphan. I am not sure how we are going to resolve this properly until we have to figure out why call_rcu() is taking so long to finish?

Another solution is to add aux call traces for both skb_dst_drop() and skb_dst_set() for this case, but that there are many places to free memory via call_rcu() like inode free etc.