net: net_failover: Fix the deadlock in net_failover_slave_name_change()

This is a sibling fix of commit
b84c5632c7b3 ("net: net_failover: Fix the deadlock in slave register").
There is netdev_lock_ops() in the upper callers, so using netif_open()
instead of dev_open().

Call Trace:
 __schedule+0x2bb/0x650
 schedule+0x27/0xb0
 schedule_preempt_disabled+0x15/0x30
 __mutex_lock.constprop.0+0x550/0xaf0
 __mutex_lock_slowpath+0x13/0x20
 mutex_lock+0x3b/0x50
 dev_open+0x3b/0xe0
 net_failover_slave_name_change+0x22/0x40
 failover_event+0xd4/0x1e0
 notifier_call_chain+0x62/0xf0
 raw_notifier_call_chain+0x16/0x30
 call_netdevice_notifiers_info+0x50/0x80
 netif_change_name+0x200/0x330
 do_setlink.isra.0+0xb12/0xdf0
 ? security_capable+0x9a/0x1e0
 ? ns_capable+0x31/0x60
 rtnl_setlink+0x302/0x670
 ? netlink_recvmsg+0x296/0x340
 ? security_capable+0x9a/0x1e0
 ? __pfx_rtnl_setlink+0x10/0x10
 rtnetlink_rcv_msg+0x384/0x460
 ? __pfx_rtnetlink_rcv_msg+0x10/0x10
 netlink_rcv_skb+0x61/0x120
 rtnetlink_rcv+0x15/0x30
 netlink_unicast+0x28f/0x3c0
 netlink_sendmsg+0x216/0x450
 __sys_sendto+0x222/0x230
 __x64_sys_sendto+0x24/0x40
 x64_sys_call+0x1d5d/0x2390
 do_syscall_64+0x105/0x5a0
 ? do_syscall_64+0x140/0x5a0
 ? exc_page_fault+0x94/0x1e0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: 7e4d784f5810 ("net: hold netdev instance lock during rtnetlink operations")
Signed-off-by: Faicker Mo <faicker.mo@gmail.com>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Link: https://patch.msgid.link/20260908040708.3972058-1-faicker.mo@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>