RDMA/rxe: Fill in wc byte_len with IB_WC_RECV_RDMA_WITH_IMM

Calculate the correct byte_len on the receiving side when a work
completion is generated with IB_WC_RECV_RDMA_WITH_IMM opcode.

According to the IBA byte_len must indicate the number of written bytes,
whereas it was always equal to zero for the IB_WC_RECV_RDMA_WITH_IMM
opcode, even though data was transferred.

Fixes: 8700e3e7

 ("Soft RoCE driver")
Signed-off-by: Konstantin Taranov <konstantin.taranov@inf.ethz.ch>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>