RDMA/mlx4: Don't crash machine if zap_vma_ptes() fails

The failure reported by zap_vma_ptes() means that wrong VMA pages
were supplied, however it is impossible for this type of address.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>