IB/IPoIB: Enqueue separate work_structs for each flushed interface

IB/IPoIB: Enqueue separate work_structs for each flushed interface

Previously, flushing a netdevice involved first flushing all child
devices from the flush task itself. That requires holding the lock that
protects the list for the entire duration of the flush.

This poses a problem when converting from vlan_rwsem to the netdev
instance lock (next patch), because holding the parent lock while
trying to acquire a child lock makes lockdep unhappy, rightfully.

Fix this by splitting a big flush task into individual flush tasks
(all are already created in their respective ipoib_dev_priv structs)
and defining a helper function to enqueue all of them while holding the
list lock.

In ipoib_set_mac, the function is not used and the task is enqueued
directly, because in the subsequent patches locking is changed and this
function may be called with the netdev instance lock held.

This is effectively a noop, the wq is single-threaded and ordered and
will execute the same flush operations in the same order as before.

Furthermore, there should be no new races because
ipoib_parent_unregister_pre() calls flush_workqueue() after stopping new
work generation to wait for pending work to complete. flush_workqueue()
waits for all currently enqueued work to finish before returning.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/1747829342-1018757-2-git-send-email-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>