net: devlink: remove region snapshots list dependency on devlink->lock

After mlx4 driver is converted to do locked reload,
devlink_region_snapshot_create() may be called from both locked and
unlocked context.

Note that in mlx4 region snapshots could be created on any command
failure. That can happen in any flow that involves commands to FW,
which means most of the driver flows.

So resolve this by removing dependency on devlink->lock for region
snapshots list consistency and introduce new mutex to ensure it.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>