complete boot failure in 4.5-rc1 caused by nvme: make SG_IO support optional

[Christoph Hellwig]

Keith said it should be on by default, and I promised him to change
it once we run into problems, which I guess this counts as.

But just curious:  what distro are you using?  Upstream systemd
explicitly rejected using scsi_id for NVMe here:

	https://github.com/systemd/systemd/issues/1453

and all my test systems don't do this either.