nvme: Apply cache operations on the DMA buffers

So far cache operations are only applied on the submission queue and
completion queue, but they are missing in other places like identify
and block read/write routines.

In order to correctly operate on the caches, the DMA buffer passed
to identify routine must be allocated properly on the stack with the
existing macro ALLOC_CACHE_ALIGN_BUFFER().

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>