NVMe: Fix sparse warnings in scsi emulation

Sparse produced warnings for some instances of
mismatched types and direct userspace dereferences.
This patch fixes those for the scsi emulation layer.

Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>