devlink: hold region lock when flushing snapshots

Netdevsim triggers a splat on reload, when it destroys regions
with snapshots pending:

  WARNING: CPU: 1 PID: 787 at net/core/devlink.c:6291 devlink_region_snapshot_del+0x12e/0x140
  CPU: 1 PID: 787 Comm: devlink Not tainted 6.1.0-07460-g7ae9888d6e1c #580
  RIP: 0010:devlink_region_snapshot_del+0x12e/0x140
  Call Trace:
   <TASK>
   devl_region_destroy+0x70/0x140
   nsim_dev_reload_down+0x2f/0x60 [netdevsim]
   devlink_reload+0x1f7/0x360
   devlink_nl_cmd_reload+0x6ce/0x860
   genl_family_rcv_msg_doit.isra.0+0x145/0x1c0

This is the locking assert in devlink_region_snapshot_del(),
we're supposed to be holding the region->snapshot_lock here.

Fixes: 2dec18ad

 ("net: devlink: remove region snapshots list dependency on devlink->lock")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>