LU-9859 libcfs: Fix using smp_processor_id() in preemptible context

LU-9859 libcfs: Fix using smp_processor_id() in preemptible context

This warning show up with kernels that enable preemptible
BUG: using smp_processor_id() in preemptible [00000000] code: ...

Change it to disable preemption around smp_processor_id().

Change is apart of:
Linux-commit: 67bc8c33ec14f8290c6883a7d6237e213709561a

Change-Id: I41f7a1d3aa22240d3669f94ae92a192d219cca52
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/38810
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>