sched/cpufreq_sched: fix thermal capping events

sched/cpufreq_sched: fix thermal capping events

cpufreq_sched_limits (called when CPUFREQ_GOV_LIMITS event happens)
bails out if policy->rwsem is already locked. However, that rwsem is
always guaranteed to be locked when we get here after a thermal
throttling event happens:

 th_throttling ->
   cpufreq_update_policy()
     ...
     down_write(&policy->rwsem);
     ...
     cpufreq_set_policy() ->
       ...
       __cpufreq_governor(policy, CPUFREQ_GOV_LIMITS); ->
         cpufreq_sched_limits()
         ...
         if (!down_write_trylock(&policy->rwsem))
                 return; <-- BAIL OUT!

So, we don't currently react immediately to thermal capping event (even
if reaction is still quick in practice, ~1ms, as lots of events are likely
to trigger a frequency selection on a high loaded system).

Fix this bug by removing the bail out condition.

While we are at it we also slightly change handling of the new limits by
clamping the last requested_freq between policy's max and min. Doing so
gives us the oppurtunity to correctly restore the last requested
frequency as soon as a thermal unthrottling event happens.

bug: 30481949

Change-Id: I3c13e818f238c1ffa66b34e419e8b87314b57427
Suggested-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Srinath Sridharan <srinathsr@google.com>
[jstultz: fwdported to 4.4]
Signed-off-by: John Stultz <john.stultz@linaro.org>