RDMA/core: Verify that memory window type is legal

Before calling the provider's alloc_mw function, verify that the
given memory type is either IB_MW_TYPE_1 or IB_MW_TYPE_2.

Signed-off-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>