context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK

Historically, context tracking had to deal with fragile entry code path,
ie: before user_exit() is called and after user_enter() is called, in
case some of those spots would call schedule() or use RCU. On such
cases, the site had to be protected between exception_enter() and
exception_exit() that save the context tracking state in the task stack.

Such sleepable fragile code path had many different origins: tracing,
exceptions, early or late calls to context tracking on syscalls...

Aside of that not being pretty, saving the context tracking state on
the task stack forces us to run context tracking on all CPUs, including
housekeepers, and prevents us to completely shutdown nohz_full at
runtime on a CPU in the future as context tracking and its overhead
would still need to run system wide.

Now thanks to the extensive efforts to sanitize x86 entry code, those
conditions have been removed and we can now get rid of these workarounds
in this architecture.

Create a Kconfig feature to express this achievement.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201117151637.259084-2-frederic@kernel.org