RDMA/rxe: Fix extra copy in prepare_ack_packet

Currently prepare_ack_packet writes almost all the fields of the BTH in
the ack packet twice. Replace code with the subroutine init_bth().

Fixes: 8700e3e7 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20210618045742.204195-6-rpearsonhpe@gmail.com


Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>