nvme-pci: fix host memory buffer allocation fallback

nvme_alloc_host_mem currently contains two loops that are interwinded,
and the outer retry loop turns out to be broken.  Fix this by untangling
the two.

Based on a report an initial patch from Akinobu Mita.

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