IB/mad: Issue complete whenever decrements agent refcount

Replace calls of atomic_dec() to mad_agent_priv->refcount with calls to
deref_mad_agent() in order to issue complete. Most likely the refcount is
> 1 at these points, but it is difficult to prove. Performance is not
important on these paths, so be obviously correct.

Link: https://lore.kernel.org/r/20200621104738.54850-3-leon@kernel.org


Signed-off-by: Shay Drory <shayd@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>