nvme-tcp: Fix warning with CONFIG_DEBUG_PREEMPT

We shouldn't call smp_processor_id() in a preemptible
context, but this is advisory at best, so instead
call __smp_processor_id().

Fixes: db5ad6b7

 ("nvme-tcp: try to send request in queue_rq context")
Reported-by: Or Gerlitz <gerlitz.or@gmail.com>
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>