IB/rxe: Do not hide uABI stuff in memcpy

struct rxe_global_route and struct ib_global_route are not the same thing
and should not be memcpy'd over each other, do a member by member copy
instead. This allows the layout of the in-kernel struct ib_global_route to
be changed without breaking rxe.

Reviewed-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>