RDMA/rvt: Do not use a kernel header in the ABI

rvt was using ib_sge as part of it's ABI, which is not allowed. Introduce
a new struct with the same layout and use it instead.

Fixes: dabac6e4 ("IB/hfi1: Move receive work queue struct into uapi directory")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>