bonding: fix missed rcu protection

bonding: fix missed rcu protection

When removing the rcu_read_lock in bond_ethtool_get_ts_info() as
discussed [1], I didn't notice it could be called via setsockopt,
which doesn't hold rcu lock, as syzbot pointed:

  stack backtrace:
  CPU: 0 PID: 3599 Comm: syz-executor317 Not tainted 5.18.0-rc5-syzkaller-01392-g01f4685797a5 #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  Call Trace:
   <TASK>
   __dump_stack lib/dump_stack.c:88 [inline]
   dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
   bond_option_active_slave_get_rcu include/net/bonding.h:353 [inline]
   bond_ethtool_get_ts_info+0x32c/0x3a0 drivers/net/bonding/bond_main.c:5595
   __ethtool_get_ts_info+0x173/0x240 net/ethtool/common.c:554
   ethtool_get_phc_vclocks+0x99/0x110 net/ethtool/common.c:568
   sock_timestamping_bind_phc net/core/sock.c:869 [inline]
   sock_set_timestamping+0x3a3/0x7e0 net/core/sock.c:916
   sock_setsockopt+0x543/0x2ec0 net/core/sock.c:1221
   __sys_setsockopt+0x55e/0x6a0 net/socket.c:2223
   __do_sys_setsockopt net/socket.c:2238 [inline]
   __se_sys_setsockopt net/socket.c:2235 [inline]
   __x64_sys_setsockopt+0xba/0x150 net/socket.c:2235
   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
   do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
   entry_SYSCALL_64_after_hwframe+0x44/0xae
  RIP: 0033:0x7f8902c8eb39

Fix it by adding rcu_read_lock and take a ref on the real_dev.
Since dev_hold() and dev_put() can take NULL these days, we can
skip checking if real_dev exist.

[1] https://lore.kernel.org/netdev/27565.1642742439@famine/

Reported-by: syzbot+92beb3d46aab498710fa@syzkaller.appspotmail.com
Fixes: aa6034678e87 ("bonding: use rcu_dereference_rtnl when get bonding active slave")
Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220519020148.1058344-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>