RDMA/rxe: Fix redundant call to ip_send_check

For IPV4 packets sent on the wire the rxe driver calls ip_local_out()
which immediately calls __ip_local_out() which sets iph->tot_len and calls
ip_send_check(). This code is duplicated in prepare4(). On the loopback
path the IP header checksum and tot_len fields are not used so they do not
need to be set.

Remove this redundant code.

Fixes: 8700e3e7

 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20210618045742.204195-3-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>