RDMA/rw: map P2P memory correctly for signature operations

Since RDMA rw API support operations with P2P memory sg list, make sure to
map/unmap the scatter list for signature operation correctly.

Link: https://lore.kernel.org/r/20200220100819.41860-2-maxg@mellanox.com
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>