The vunmap() function performs 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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/b3fbdcaf-1bda-7ce9-935b-2d716727ec39@users.sourceforge.net