kcm: avoid potential race in kcm_tx_work

syzbot found that kcm_tx_work() could crash [1] in:

	/* Primarily for SOCK_SEQPACKET sockets */
	if (likely(sk->sk_socket) &&
	    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
<<*>>	clear_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
		sk->sk_write_space(sk);
	}

I think the reason is that another thread might concurrently
run in kcm_release() and call sock_orphan(sk) while sk is not
locked. kcm_tx_work() find sk->sk_socket being NULL.

[1]
BUG: KASAN: null-ptr-deref in instrument_atomic_write include/linux/instrumented.h:86 [inline]
BUG: KASAN: null-ptr-deref in clear_bit include/asm-generic/bitops/instrumented-atomic.h:41 [inline]
BUG: KASAN: null-ptr-deref in kcm_tx_work+0xff/0x160 net/kcm/kcmsock.c:742
Write of size 8 at addr 0000000000000008 by task kworker/u4:3/53

CPU: 0 PID: 53 Comm: kworker/u4:3 Not tainted 5.19.0-rc3-next-20220621-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: kkcmd kcm_tx_work
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
kasan_report+0xbe/0x1f0 mm/kasan/report.c:495
check_region_inline mm/kasan/generic.c:183 [inline]
kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189
instrument_atomic_write include/linux/instrumented.h:86 [inline]
clear_bit include/asm-generic/bitops/instrumented-atomic.h:41 [inline]
kcm_tx_work+0xff/0x160 net/kcm/kcmsock.c:742
process_one_work+0x996/0x1610 kernel/workqueue.c:2289
worker_thread+0x665/0x1080 kernel/workqueue.c:2436
kthread+0x2e9/0x3a0 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:302
</TASK>

Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Link: https://lore.kernel.org/r/20221012133412.519394-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
