ext4: fix suspicious RCU usage warning in ext4_release_system_zone

hulk inclusion
category: bugfix
bugzilla: 18685
CVE: NA

-----------------------------

The rcu_dereference() should be used under rcu_read_lock(), or else it
will complain about it may be a suspicious RCU usage.

 WARNING: suspicious RCU usage
 [...]
 -----------------------------
 fs/ext4/block_validity.c:331 suspicious rcu_dereference_check() usage!
 [...]

Because ext4_release_system_zone() always under protection of
sb->s_umount, so the proper fix is switch to use
rcu_dereference_protected() instead.

Fixes: fb9fd3ade129be ("ext4: fix potential use after free in system zone via remount with noblock_validity")
Reviewed-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Yi Zhang <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>