RDMA/siw: Cleanup unused mmap structures.

Removes obsolete driver specific mmap information after
generalization of RDMA driver mmap service. Also removes
useless forward declaration of struct siw_mr.

Fixes: 11f1a755 ("RDMA/siw: Use the common mmap_xa helpers")
Link: https://lore.kernel.org/r/20191113153404.7402-1-bmt@zurich.ibm.com


Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>