RDMA/uverbs: Use uverbs_attr_bundle to pass ucore for write/write_ex

This creates a consistent way to access the two core buffers across write
and write_ex handlers.

Remove the open coded ucore conversion in the write/ex compatibility
handlers.

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