This patch implements and enables context tracking for riscv (which is a
prerequisite for CONFIG_NO_HZ_FULL support)

It adds checking for previous state in the entry that all excepttions and
interrupts goes to and calls context_tracking_user_exit() if it comes from
user space. It also calls context_tracking_user_enter() if it will return
to user space before restore_all.

This patch is tested with the dynticks-testing testcase in
qemu-system-riscv64 virt machine and Unleashed board.
git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git

We can see the log here. The tick got mostly stopped during the execution
of the user loop.

                        _-----=> irqs-off
                       / _----=> need-resched
                      | / _---=> hardirq/softirq
                      || / _--=> preempt-depth
                      ||| /     delay
     TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
        | |       |   ||||       |         |
   <idle>-0     [001] d..2   604.183512: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=taskset next_pid=273 next_prio=120
user_loop-273   [001] d.h1   604.184788: hrtimer_expire_entry: hrtimer=000000002eda5fab function=tick_sched_timer now=604176096300
user_loop-273   [001] d.s2   604.184897: workqueue_queue_work: work struct=00000000383402c2 function=vmstat_update workqueue=00000000f36d35d4 req_cpu=1 cpu=1
user_loop-273   [001] dns2   604.185039: tick_stop: success=0 dependency=SCHED
user_loop-273   [001] dn.1   604.185103: tick_stop: success=0 dependency=SCHED
user_loop-273   [001] d..2   604.185154: sched_switch: prev_comm=taskset prev_pid=273 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=46 next_prio=120
    <...>-46    [001] ....   604.185194: workqueue_execute_start: work struct 00000000383402c2: function vmstat_update
    <...>-46    [001] d..2   604.185266: sched_switch: prev_comm=kworker/1:1 prev_pid=46 prev_prio=120 prev_state=I ==> next_comm=taskset next_pid=273 next_prio=120
user_loop-273   [001] d.h1   604.188812: hrtimer_expire_entry: hrtimer=000000002eda5fab function=tick_sched_timer now=604180133400
user_loop-273   [001] d..1   604.189050: tick_stop: success=1 dependency=NONE
user_loop-273   [001] d..2   614.251386: sched_switch: prev_comm=user_loop prev_pid=273 prev_prio=120 prev_state=X ==> next_comm=swapper/1 next_pid=0 next_prio=120
   <idle>-0     [001] d..2   614.315391: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=taskset next_pid=276 next_prio=120
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>