sparc/sysfs: Replace racy task affinity logic

The mmustat_enable sysfs file accessor functions must run code on the
target CPU. This is achieved by temporarily setting the affinity of the
calling user space thread to the requested CPU and reset it to the original
affinity afterwards.

That's racy vs. concurrent affinity settings for that thread resulting in
code executing on the wrong CPU and overwriting the new affinity setting.

Replace it by using work_on_cpu() which guarantees to run the code on the
requested CPU.

Protection against CPU hotplug is not required as the open sysfs file
already prevents the removal from the CPU offline callback. Using the
hotplug protected version would actually be wrong because it would deadlock
against a CPU hotplug operation of the CPU associated to the sysfs file in
progress.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: fenghua.yu@intel.com
Cc: tony.luck@intel.com
Cc: herbert@gondor.apana.org.au
Cc: rjw@rjwysocki.net
Cc: peterz@infradead.org
Cc: benh@kernel.crashing.org
Cc: bigeasy@linutronix.de
Cc: jiangshanlai@gmail.com
Cc: sparclinux@vger.kernel.org
Cc: viresh.kumar@linaro.org
Cc: mpe@ellerman.id.au
Cc: tj@kernel.org
Cc: lenb@kernel.org
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1704131001270.2408@nanos


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>