NVMe: Fix endian-related problems in user I/O submission path

When constructing the command, dsmgmt needs to be treated as a 32-bit
value, not a 16-bit value.  reftag, apptag and appmask all need to be
converted from native-endian to little-endian.  Again, sparse's bitwise
warnings caught this problem.  Thanks to Keith for pointing out the
correct way to fix the reftag.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Acked-by: Keith Busch <keith.busch@intel.com>