tipc: Fix missing RTNL lock protection during setting link properties

Currently when user changes link properties, TIPC first checks if
user's command message contains media name or bearer name through
tipc_media_find() or tipc_bearer_find() which is protected by RTNL
lock. But when tipc_nl_compat_link_set() conducts the checking with
the two functions, it doesn't hold RTNL lock at all, as a result,
the following complaints were reported:

audit: type=1400 audit(1514679888.244:9): avc:  denied  { write } for
pid=3194 comm="syzkaller021477" path="socket:[11143]" dev="sockfs"
ino=11143 scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
tcontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
tclass=netlink_generic_socket permissive=1
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>

=============================
WARNING: suspicious RCU usage
4.15.0-rc5+ #152 Not tainted
-----------------------------
net/tipc/bearer.c:177 suspicious rcu_dereference_protected() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
2 locks held by syzkaller021477/3194:
  #0:  (cb_lock){++++}, at: [<00000000d20133ea>] genl_rcv+0x19/0x40
net/netlink/genetlink.c:634
  #1:  (genl_mutex){+.+.}, at: [<00000000fcc5d1bc>] genl_lock
net/netlink/genetlink.c:33 [inline]
  #1:  (genl_mutex){+.+.}, at: [<00000000fcc5d1bc>] genl_rcv_msg+0x115/0x140
net/netlink/genetlink.c:622

stack backtrace:
CPU: 1 PID: 3194 Comm: syzkaller021477 Not tainted 4.15.0-rc5+ #152
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x257 lib/dump_stack.c:53
  lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4585
  tipc_bearer_find+0x2b4/0x3b0 net/tipc/bearer.c:177
  tipc_nl_compat_link_set+0x329/0x9f0 net/tipc/netlink_compat.c:729
  __tipc_nl_compat_doit net/tipc/netlink_compat.c:288 [inline]
  tipc_nl_compat_doit+0x15b/0x660 net/tipc/netlink_compat.c:335
  tipc_nl_compat_handle net/tipc/netlink_compat.c:1119 [inline]
  tipc_nl_compat_recv+0x112f/0x18f0 net/tipc/netlink_compat.c:1201
  genl_family_rcv_msg+0x7b7/0xfb0 net/netlink/genetlink.c:599
  genl_rcv_msg+0xb2/0x140 net/netlink/genetlink.c:624
  netlink_rcv_skb+0x21e/0x460 net/netlink/af_netlink.c:2408
  genl_rcv+0x28/0x40 net/netlink/genetlink.c:635
  netlink_unicast_kernel net/netlink/af_netlink.c:1275 [inline]
  netlink_unicast+0x4e8/0x6f0 net/netlink/af_netlink.c:1301
  netlink_sendmsg+0xa4a/0xe60 net/netlink/af_netlink.c:1864
  sock_sendmsg_nosec net/socket.c:636 [inline]
  sock_sendmsg+0xca/0x110 net/socket.c:646
  sock_write_iter+0x31a/0x5d0 net/socket.c:915
  call_write_iter include/linux/fs.h:1772 [inline]
  new_sync_write fs/read_write.c:469 [inline]
  __vfs_write+0x684/0x970 fs/read_write.c:482
  vfs_write+0x189/0x510 fs/read_write.c:544
  SYSC_write fs/read_write.c:589 [inline]
  SyS_write+0xef/0x220 fs/read_write.c:581
  do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
  do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
  entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:129

In order to correct the mistake, __tipc_nl_compat_doit() has been
protected by RTNL lock, which means the whole operation of setting
bearer/media properties is under RTNL protection.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reported-by: syzbot <syzbot+6345fd433db009b29413@syzkaller.appspotmail.com>

Signed-off-by: David S. Miller <davem@davemloft.net>