block: Replace __this_cpu_ptr with raw_cpu_ptr

__this_cpu_ptr is being phased out use raw_cpu_ptr instead which was
introduced in 3.15-rc1.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>