cpufreq: Split cpufreq_offline()

[ Upstream commit fddd8f86dff4a24742a7f0322ccbb34c6c1c9850 ]

Split the "core" part running under the policy rwsem out of
cpufreq_offline() to allow the locking in cpufreq_remove_dev() to be
rearranged more easily.

As a side-effect this eliminates the unlock label that's not needed
any more.

No expected functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Stable-dep-of: b8f85833c057 ("cpufreq: exit() callback is optional")
Signed-off-by: Sasha Levin <sashal@kernel.org>