syzkaller triggered the warning [0] in udp_v4_early_demux().
In udp_v[46]_early_demux() and sk_lookup(), we do not touch the refcount
of the looked-up sk and use sock_pfree() as skb->destructor, so we check
SOCK_RCU_FREE to ensure that the sk is safe to access during the RCU grace
period.
Currently, SOCK_RCU_FREE is flagged for a bound socket after being put
into the hash table. Moreover, the SOCK_RCU_FREE check is done too early
in udp_v[46]_early_demux() and sk_lookup(), so there could be a small race
window: