RDMA/mlx5: Remove duplicate pointer assignment in mlx5_ib_alloc_implicit_mr()

The pointer imr->umem is assigned twice. Fix this by removing the
redundant one.

Link: https://lore.kernel.org/r/20220518044914.1903125-1-matsuda-daisuke@fujitsu.com


Signed-off-by: Daisuke Matsuda <matsuda-daisuke@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>