NIC is stopped with rtnl_lock held, and during the stop it cancels the
'service_task' work and free irqs.

However, if CONFIG_MACSEC is set, rtnl_lock is acquired both from
aq_nic_service_task and aq_linkstate_threaded_isr. Then a deadlock
happens if aq_nic_stop tries to cancel/disable them when they've already
started their execution.

As the deadlock is caused by rtnl_lock, it causes many other processes
to stall, not only atlantic related stuff.

Fix it by introducing a mutex that protects each NIC's macsec related
data, and locking it instead of the rtnl_lock from the service task and
the threaded IRQ.

Before this patch, all macsec data was protected with rtnl_lock, but
maybe not all of it needs to be protected. With this new mutex, further
efforts can be made to limit the protected data only to that which
requires it. However, probably it doesn't worth it because all macsec's
data accesses are infrequent, and almost all are done from macsec_ops
or ethtool callbacks, called holding rtnl_lock, so macsec_mutex won't
never be much contended.

The issue appeared repeteadly attaching and deattaching the NIC to a
bond interface. Doing that after this patch I cannot reproduce the bug.

Fixes: 62c1c2e6 ("net: atlantic: MACSec offload skeleton")
Reported-by: Li Liang <liali@redhat.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Reviewed-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>