MIPS,bpf: Fix using smp_processor_id() in preemptible splat.

If the kernel is configured with preemption enabled we were getting
warning stack traces for use of current_cpu_type().

Fix by moving the test between preempt_disable()/preempt_enable() and
caching the results of the CPU type tests for use during code
generation.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>