drivers/zram: fix zcomp_stream_get() smp_processor_id() use in preemptible code

Use get_local_ptr() instead this_cpu_ptr() to avoid a warning regarding
smp_processor_id() in preemptible code.
raw_cpu_ptr() would be fine, too because the per-CPU data structure is
protected with a spin lock so it does not matter much if we take the
other one.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>