[MIPS] Fix use of smp_processor_id() in preemptible code.

Freeing prom memory: 956kb freed
Freeing firmware memory: 978944k freed
Freeing unused kernel memory: 180k freed
BUG: using smp_processor_id() in preemptible [00000000] code: swapper/1
caller is r4k_dma_cache_wback_inv+0x144/0x2a0
Call Trace:
 [<80117af8>] r4k_dma_cache_wback_inv+0x144/0x2a0
 [<802e4b84>] debug_smp_processor_id+0xd4/0xf0
 [<802e4b7c>] debug_smp_processor_id+0xcc/0xf0
...
CONFIG_DEBUG_PREEMPT is enabled.
--
Bug cause is blast_dcache_range() in preemptible code [in
r4k_dma_cache_wback_inv()].
blast_dcache_range() is constructed via __BUILD_BLAST_CACHE_RANGE that
uses cpu_dcache_line_size(). It uses current_cpu_data that use
smp_processor_id() in turn. In case of CONFIG_DEBUG_PREEMPT
smp_processor_id emits BUG if we are executing with preemption
enabled.

Cpu options of cpu0 are assumed to be the superset of all processors.

Can I make the same assumptions for cache line size  and fix this
issue the following way:

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>