NVMe: Fix DMA mapping for admin commands

We were always mapping as DMA_FROM_DEVICE then unmapping with
DMA_TO_DEVICE which was clearly not correct.  Follow the same pattern as
nvme_submit_io() and key off the bottom bit of the opcode to determine
whether this is a read or a write.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>