tipc: check return value of __tipc_dump_start()

When __tipc_dump_start() fails with running out of memory,
we have no reason to continue, especially we should avoid
calling tipc_dump_done().

Fixes: 8f5c5fcf ("tipc: call start and done ops directly in __tipc_nl_compat_dumpit()")
Reported-and-tested-by:  <syzbot+3f8324abccfbf8c74a9f@syzkaller.appspotmail.com>
Cc: Jon Maloy <jon.maloy@ericsson.com>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>