ocfs2: fix slab-use-after-free due to dangling pointer dqi_priv

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

ocfs2: fix slab-use-after-free due to dangling pointer dqi_priv

When mounting ocfs2 and then remounting it as read-only, a

slab-use-after-free occurs after the user uses a syscall to

quota_getnextquota. Specifically, sb_dqinfo(sb, type)->dqi_priv is the

dangling pointer.

During the remounting process, the pointer dqi_priv is freed but is never

set as null leaving it to be accessed. Additionally, the read-only option

for remounting sets the DQUOT_SUSPENDED flag instead of setting the

DQUOT_USAGE_ENABLED flags. Moreover, later in the process of getting the

next quota, the function ocfs2_get_next_id is called and only checks the

quota usage flags and not the quota suspended flags.

To fix this, I set dqi_priv to null when it is freed after remounting with

read-only and put a check for DQUOT_SUSPENDED in ocfs2_get_next_id.

[[email protected]: coding-style cleanups]