psref leak detector (aggressive version)

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

https://www.netbsd.org/~ozaki-r/psref-debug-aggressive.diff

This patch implements yet another psref leak detector that
enables to tell where a leak occurs while a simpler version
that is already committed just tells an occurrence of a leak.

Investigating of psref leaks is hard because once a leak occurs
a percpu list of psref that tracks references can be corrupted.
A reference to a tracking object is memorized in the list
via an intermediate object (struct psref) that is normally allocated
on a stack of a thread.  Thus, the intermediate object can be
overwritten on a leak resulting in corrupt of the list.

The tracker makes a shadow entry to an intermediate object and
stores some hints into it (currently it's a caller address of
psref_acquire).  We can detect a leak by checking the entries on
certain points where any references should be released such as
the return point of syscalls and the end of each softint handler.

Note that the current version is based on -current.  Once the
simpler version is revised as using lwp_specificdata [*], the
patch will be tweaked too.

[*] http://mail-index.netbsd.org/source-changes-d/2019/05/08/msg011297.html

Any comments?

Thanks,
  ozaki-r