The vunmap() function performes also input parameter validation. Thus the test
around the call is not needed.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>