RDMA/rxe: Remove the repeated 'mr->umem = umem'

Drop duplicated code

Link: https://lore.kernel.org/r/20210702123024.37025-1-ice_yangxiao@163.com


Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>