xen-gntdev: Use map->vma for checking map validity

The is_mapped flag used to be set at the completion of the map operation,
but was not checked in all error paths. Use map->vma instead, which will
now be cleared if the initial grant mapping fails.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>