RDMA/rxe: Fix extra copies in build_rdma_network_hdr

build_rdma_network_hdr() in rxe_resp.c does more copying than is
needed. Remove this subroutine and eliminate the extra copies for IPV6 and
reduce the extra copying for IPV4.

Fixes: e404f945

 ("IB/rxe: improved debug prints & code cleanup")
Link: https://lore.kernel.org/r/20210618045742.204195-4-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>