net: ethtool: keep rtnl_lock for ops using ethtool_op_get_link()

net: ethtool: keep rtnl_lock for ops using ethtool_op_get_link()

Breno reports following splats on mlx5:

  RTNL: assertion failed at net/core/dev.c (2241)
  WARNING: net/core/dev.c:2241 at netif_state_change+0xed/0x130, CPU#5: ethtool/1335
  RIP: 0010:netif_state_change+0xf9/0x130
  Call Trace:
    <TASK>
     __linkwatch_sync_dev+0xea/0x120
     ethtool_op_get_link+0xe/0x20
     __ethtool_get_link+0x26/0x40
     linkstate_prepare_data+0x51/0x200
     ethnl_default_doit+0x213/0x470
     genl_family_rcv_msg_doit+0xdd/0x110

Looks like I missed ethtool_op_get_link() trying to sync linkwatch,
which needs rtnl_lock. Not all drivers do this - bnxt doesn't,
it just returns the link state, so add an opt-in bit.

Reported-by: Breno Leitao <leitao@debian.org>
Fixes: 45079e00133e ("net: ethtool: optionally skip rtnl_lock on Netlink path for GET ops")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Breno Leitao <leitao@debian.org>
Acked-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20260624190439.2521219-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>