IB/mlx4: Avoid executing gid task when device is being removed

IB/mlx4: Avoid executing gid task when device is being removed

commit 4bf9715f184969dc703bde7be94919995024a6a9 upstream.

When device is being removed (e.g during VPI port link type change
from ETH to IB), tasks for gid table changes should not be executed.

Flush the current queue of tasks and block further tasks from entering the queue.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>