->retries fixups

[Christoph Hellwig]

This series fixes a few lose bits in terms of how nvme uses ->retries,
including fixing it for non-PCIe transports.  While at it I noticed that
nvme and scsi use the field in entirely different ways, and no other
driver uses it at all.  So I decided to move it into the nvme_request and
scsi_request structures instead.