usbip: Replace unused kvec array with single variable in vhci_send_cmd_unlink()

vhci_send_cmd_unlink() declears kvec array of size 3 but it actually
uses just one element of the array. So, remove kvec array and replace
it with single kvec variable.

Signed-off-by: Suwan Kim <suwan.kim027@gmail.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>