IB/hfi1: constify vm_operations_struct

vm_operations_struct are not supposed to change at runtime.
vm_area_struct structure working with const vm_operations_struct.
So mark the non-const vm_operations_struct structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>