Description of problem:
`gnbd_export -r` doesn't verify that there is an argument passed to
the remove option.  According to Ben Marzinski, if there is no GNBD
device listed when '-r' is specified, gnbd_export should error.

Version-Release number of selected component (if applicable):
GFS-6.0.0-15 and earlier

How reproducible:
Always

Steps to Reproduce:
1. gnbd_export -r ; echo $?

  
Actual results:
It prints no error and returns with an exit status of 0

Expected results:
It should print an error indicating that the command is missing an
argument and fail with an exit status of 1.