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.org


Signed-off-by: Leon Romanovsky <leon@kernel.org>