netfilter: ipset: fix suspicious RCU usage in find_set_and_id

find_set_and_id() is called when the NFNL_SUBSYS_IPSET mutex is held.
However, in the error path there can be a follow-up recvmsg() without
the mutex held. Use the start() function of struct netlink_dump_control
instead of dump() to verify and report if the specified set does not
exist.

Thanks to Pablo Neira Ayuso for helping me to understand the subleties
of the netlink protocol.

Reported-by:  <syzbot+fc69d7cb21258ab4ae4d@syzkaller.appspotmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>