sched: etf: do not assume all sockets are full blown

skb->sk does not always point to a full blown socket,
we need to use sk_fullsock() before accessing fields which
only make sense on full socket.

BUG: KASAN: use-after-free in report_sock_error+0x286/0x300 net/sched/sch_etf.c:141
Read of size 1 at addr ffff88805eb9b245 by task syz-executor.5/9630

CPU: 1 PID: 9630 Comm: syz-executor.5 Not tainted 5.7.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xd3/0x315 mm/kasan/report.c:382
 __kasan_report.cold+0x35/0x4d mm/kasan/report.c:511
 kasan_report+0x33/0x50 mm/kasan/common.c:625
 report_sock_error+0x286/0x300 net/sched/sch_etf.c:141
 etf_enqueue_timesortedlist+0x389/0x740 net/sched/sch_etf.c:170
 __dev_xmit_skb net/core/dev.c:3710 [inline]
 __dev_queue_xmit+0x154a/0x30a0 net/core/dev.c:4021
 neigh...