igbvf: fix issue w/ mapped_as_page being left set after unmap

This change fixes an issue in igbvf with mapped_as_page being left set
after a page is unmapped which results in buffers which are mapped via map
single being unmapped as page.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>