RDMA/mlx5: Add missing returned error check of mlx5_ib_dereg_mr

Fix the following smatch error:

drivers/infiniband/hw/mlx5/mr.c:1950 mlx5_ib_dereg_mr() error: uninitialized symbol 'rc'.

Fixes: e6fb246c ("RDMA/mlx5: Consolidate MR destruction to mlx5_ib_dereg_mr()")
Link: https://lore.kernel.org/r/20210314082250.10143-1-leon@kernel.org


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>