Add an assert to verify that the device lock is always held throughout
reload operations.

Tested the following flows with netdevsim and mlxsw while lockdep is
enabled:

netdevsim:

 # echo "10 1" > /sys/bus/netdevsim/new_device
 # devlink dev reload netdevsim/netdevsim10
 # ip netns add bla
 # devlink dev reload netdevsim/netdevsim10 netns bla
 # ip netns del bla
 # echo 10 > /sys/bus/netdevsim/del_device

mlxsw:

 # devlink dev reload pci/0000:01:00.0
 # ip netns add bla
 # devlink dev reload pci/0000:01:00.0 netns bla
 # ip netns del bla
 # echo 1 > /sys/bus/pci/devices/0000\:01\:00.0/remove
 # echo 1 > /sys/bus/pci/rescan

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>