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

In the Linux kernel, the following vulnerability has been resolved:

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

This is a follow-up for commit 974cb0e3e7c9 ("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]