Add an open_dev() helper function to be used in the various places where nvmecontrol needs to open a handle to a controller or namespace device node.
Add an open_dev() helper function to be used in the various places
where nvmecontrol needs to open a handle to a controller or namespace
device node.

Sponsored by:	Intel
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase