RDMA: Clean MW allocation and free flows

Move allocation and destruction of memory windows under ib_core
responsibility and clean drivers to ensure that no updates to MW
ib_core structures are done in driver layer.

Link: https://lore.kernel.org/r/20200902081623.746359-2-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>