IB/mlx4: Fix an incorrectly shadowed variable in mlx4_ib_rereg_user_mr

This error was detected by sparse static checker:

    drivers/infiniband/hw/mlx4/mr.c:226:21: warning: symbol 'err' shadows an earlier one
    drivers/infiniband/hw/mlx4/mr.c:197:13: originally declared here

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>