cpufreq/ondemand: fix race while offlining CPU

cpufreq/ondemand: fix race while offlining CPU

Offlining a CPU involves stopping the cpufreq governor. The on-demand
governor will kill the timer before letting generic code proceed, but
since that generally isn't happening on the subject CPU,
cpufreq_dbs_timer_resume() may run in parallel. If that managed to
invoke the timer handler, that handler needs to run to completion before
dbs_timer_exit() may safely exit.

Make the "stoppable" field a tristate, changing it from +1 to -1 around
the timer function invocation, and make dbs_timer_exit() wait for it to
become non-negative (still writing zero if it's +1).

Also adjust coding style in cpufreq_dbs_timer_resume().

Reported-by: Martin Cerveny <martin@c-home.cz>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Martin Cerveny <martin@c-home.cz>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>