tipc: fix the msg->req tlv len check in tipc_nl_compat_name_table_dump_header

This is a follow-up for commit 974cb0e3

 ("tipc: fix uninit-value
in tipc_nl_compat_name_table_dump") where it should have type casted
sizeof(..) to int to work when TLV_GET_DATA_LEN() returns a negative
value.

syzbot reported a call trace because of it:

  BUG: KMSAN: uninit-value in ...
   tipc_nl_compat_name_table_dump+0x841/0xea0 net/tipc/netlink_compat.c:934
   __tipc_nl_compat_dumpit+0xab2/0x1320 net/tipc/netlink_compat.c:238
   tipc_nl_compat_dumpit+0x991/0xb50 net/tipc/netlink_compat.c:321
   tipc_nl_compat_recv+0xb6e/0x1640 net/tipc/netlink_compat.c:1324
   genl_family_rcv_msg_doit net/netlink/genetlink.c:731 [inline]
   genl_family_rcv_msg net/netlink/genetlink.c:775 [inline]
   genl_rcv_msg+0x103f/0x1260 net/netlink/genetlink.c:792
   netlink_rcv_skb+0x3a5/0x6c0 net/netlink/af_netlink.c:2501
   genl_rcv+0x3c/0x50 net/netlink/genetlink.c:803
   netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
   netlink_unicast+0xf3b/0x1270 net/netlink/af_netlink.c:1345
   netlink_sendmsg+0x1288/0x1440 net/netlink/af_netlink.c:1921
   sock_sendmsg_nosec net/socket.c:714 [inline]
   sock_sendmsg net/socket.c:734 [inline]

Reported-by:  <syzbot+e5dbaaa238680ce206ea@syzkaller.appspotmail.com>
Fixes: 974cb0e3

 ("tipc: fix uninit-value in tipc_nl_compat_name_table_dump")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/ccd6a7ea801b15aec092c3b532a883b4c5708695.1667594933.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>