RDMA/rw: Introduce rdma_rw_inv_key helper

This is a preparation for adding new signature API to the rw-API.

Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>