nvme-pci: use appropriate initial chunk size for HMB allocation

The initial chunk size for host memory buffer allocation is currently
PAGE_SIZE << MAX_ORDER.  MAX_ORDER order allocation is usually failed
without CONFIG_DMA_CMA.  So the HMB allocation is retried with chunk size
PAGE_SIZE << (MAX_ORDER - 1) in general, but there is no problem if the
retry allocation works correctly.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
[hch: rebased]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Cc: stable@vger.kernel.org