net/mlx5: Nullify qp->dbg pointer post destruction

Nullifying qp->dbg is a preparation for the next patches
from the series in which mlx5_core_destroy_qp() could actually fail,
and then it can be called again which causes a kernel crash, since
qp->dbg was not nullified in previous call.
Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Link: https://lore.kernel.org/r/1677e52bb642fd8d6062d73a5aa69083c0283dc9.1685953497.git.leon@kernel.orgSigned-off-by: Leon Romanovsky <leon@kernel.org>

drivers/net/ethernet/mellanox/mlx5/core/debugfs.c

@@ -513,11 +513,11 @@ EXPORT_SYMBOL(mlx5_debug_qp_add);
 
 void mlx5_debug_qp_remove(struct mlx5_core_dev *dev, struct mlx5_core_qp *qp)
 {
-	if (!mlx5_debugfs_root)
+	if (!mlx5_debugfs_root || !qp->dbg)
 		return;
 
-	if (qp->dbg)
-		rem_res_tree(qp->dbg);
+	rem_res_tree(qp->dbg);
+	qp->dbg = NULL;
 }
 EXPORT_SYMBOL(mlx5_debug_qp_remove);