Fix incorrect error message in iovctl

If the iovctl command was invoked with only the -C flag, the user would
receive a message claiming that they needed to also supply either the
-d flag or the -f flag. However, in the case of the -C mode, only the
-f flag is acceptable. Correct this error message in this case.

PR: 222050
Submitted by: Heinz N. Gies
Reported by: Heinz N. Gies
MFC after: 1 week