md: don't dereference mddev after export_rdev()

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

md: don't dereference mddev after export_rdev()

Except for initial reference, mddev->kobject is referenced by
rdev->kobject, and if the last rdev is freed, there is no guarantee that
mddev is still valid. Hence mddev should not be used anymore after
export_rdev().

This problem can be triggered by following test for mdadm at very
low rate:

New file: mdadm/tests/23rdev-lifetime

devname=${dev0##*/}
devt=`cat /sys/block/$devname/dev`
pid=""
runtime=2

clean_up_test() {
pill -9 $pid
echo clear > /sys/block/md0/md/array_state
}

trap 'clean_up_test' EXIT

add_by_sysfs() {
while true; do
echo $devt > /sys/block/md0/md/new_dev
done
}

remove_by_sysfs(){
while true; do
echo remove > /sys/block/md0/md/dev-${devname}/state
done
}

echo md0 > /sys/module/md_mod/parameters/new_array || die "create md0 failed"

add_by_sysfs &
pid="$pid $!"

remove_by_sysfs &
pid="$pid $!"

sleep $runtime
exit 0

Test cmd:

./test --save-logs --logdir=/tmp/ --keep-going --dev=loop --tests=23rdev-lifetime

Test result:

general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6bcb: 0000 [#4] PREEMPT SMP
CPU: 0 PID: 1292 Comm: test Tainted: G D W 6.5.0-rc2-00121-g01e55c376936 #562
RIP: 0010:md_wakeup_thread+0x9e/0x320 [md_mod]
Call Trace:
<TASK>
mddev_unlock+0x1b6/0x310 [md_mod]
rdev_attr_store+0xec/0x190 [md_mod]
sysfs_kf_write+0x52/0x70
kernfs_fop_write_iter+0x19a/0x2a0
vfs_write+0x3b5/0x770
ksys_write+0x74/0x150
__x64_sys_write+0x22/0x30
do_syscall_64+0x40/0x90
entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fix this problem by don't dereference mddev after export_rdev().