RDMA/siw: Fix page address mapping in TX path

RDMA/siw: Fix page address mapping in TX path

Use the correct kmap()/kunmap() flow to determine page address used for
CRC computation. Using page_address() is wrong, since page might be in
highmem.

Fixes: b9be6f18cf9e ("rdma/siw: transmit path")
Link: https://lore.kernel.org/r/20190909132427.30264-1-bmt@zurich.ibm.com
Reported-by: Krishnamraju Eraparaju <krishna2@chelsio.com>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>