af_unix: Fix data-race around unix_tot_inflight.

In the Linux kernel, the following vulnerability has been resolved:

af_unix: Fix data-race around unix_tot_inflight.

unix_tot_inflight is changed under spin_lock(unix_gc_lock), but
unix_release_sock() reads it locklessly.

Let's use READ_ONCE() for unix_tot_inflight.

Note that the writer side was marked by commit 9d6d7f1cb67c ("af_unix:
annote lockless accesses to unix_tot_inflight & gc_in_progress")

BUG: KCSAN: data-race in unix_inflight / unix_release_sock

write (marked) to 0xffffffff871852b8 of 4 bytes by task 123 on cpu 1:
unix_inflight+0x130/0x180 net/unix/scm.c:64
unix_attach_fds+0x137/0x1b0 net/unix/scm.c:123
unix_scm_to_skb net/unix/af_unix.c:1832 [inline]
unix_dgram_sendmsg+0x46a/0x14f0 net/unix/af_unix.c:1955
sock_sendmsg_nosec net/socket.c:724 [inline]
sock_sendmsg+0x148/0x160 net/socket.c:747
____sys_sendmsg+0x4e4/0x610 net/socket.c:2493
___sys_sendmsg+0xc6/0x140 net/socket.c:2547
__sys_sendmsg+0x94/0x140 net/socket.c:2576
__do_sys_sendmsg net/socket.c:2585 [inline]
__se_sys_sendmsg net/socket.c:2583 [inline]
__x64_sys_sendmsg+0x45/0x50 net/socket.c:2583
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3b/0x90 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x72/0xdc

read to 0xffffffff871852b8 of 4 bytes by task 4891 on cpu 0:
unix_release_sock+0x608/0x910 net/unix/af_unix.c:671
unix_release+0x59/0x80 net/unix/af_unix.c:1058
__sock_release+0x7d/0x170 net/socket.c:653
sock_close+0x19/0x30 net/socket.c:1385
__fput+0x179/0x5e0 fs/file_table.c:321
____fput+0x15/0x20 fs/file_table.c:349
task_work_run+0x116/0x1a0 kernel/task_work.c:179
resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
exit_to_user_mode_prepare+0x174/0x180 kernel/entry/common.c:204
__syscall_exit_to_user_mode_work kernel/entry/common.c:286 [inline]
syscall_exit_to_user_mode+0x1a/0x30 kernel/entry/common.c:297
do_syscall_64+0x4b/0x90 arch/x86/entry/common.c:86
entry_SYSCALL_64_after_hwframe+0x72/0xdc

value changed: 0x00000000 -> 0x00000001

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 4891 Comm: systemd-coredum Not tainted 6.4.0-rc5-01219-gfa0e21fa4443 #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014