BACKPORT: FROMLIST: ovl: get_acl: Fix null pointer dereference at realinode in rcu-walk mode

Following process:
         P1                     P2
 path_openat
  link_path_walk
   may_lookup
    inode_permission(rcu)
     ovl_permission
      acl_permission_check
       check_acl
        get_cached_acl_rcu
         ovl_get_inode_acl
          realinode = ovl_inode_real(ovl_inode)
                              drop_cache
                               __dentry_kill(ovl_dentry)
                                iput(ovl_inode)
                                 ovl_destroy_inode(ovl_inode)
                                  dput(oi->__upperdentry)
                                   dentry_kill(upperdentry)
                                    dentry_unlink_inode
                                     upperdentry->d_inode = NULL
            ovl_inode_upper
             upperdentry = ovl_i_dentry_upper(ovl_inode)
             d_inode(upperdentry) // returns NULL
          IS_POSIXACL(realinode) // NULL pointer dereference
, will trigger an null pointer dereference at realinode:
  [  205.472797] BUG: kernel NULL pointer dereference, address:
                 0000000000000028
  [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
                 6.3.0-12064-g2edfa098e750-dirty #1216
  [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
  [  205.489584] Call Trace:
  [  205.489812]  <TASK>
  [  205.490014]  ovl_get_inode_acl+0x26/0x30
  [  205.490466]  get_cached_acl_rcu+0x61/0xa0
  [  205.490908]  generic_permission+0x1bf/0x4e0
  [  205.491447]  ovl_permission+0x79/0x1b0
  [  205.491917]  inode_permission+0x15e/0x2c0
  [  205.492425]  link_path_walk+0x115/0x550
  [  205.493311]  path_lookupat.isra.0+0xb2/0x200
  [  205.493803]  filename_lookup+0xda/0x240
  [  205.495747]  vfs_fstatat+0x7b/0xb0

Fetch a reproducer in [Link].

Use the helper ovl_i_path_realinode() to get realinode and then do
non-nullptr checking.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404


Fixes: 332f606b ("ovl: enable RCU'd ->get_acl()")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Suggested-by: Christian Brauner <brauner@kernel.org>
Suggested-by: Amir Goldstein <amir73il@gmail.com>

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

Bug: 315418522
Bug: 296173010
Link: https://lore.kernel.org/linux-unionfs/20230516-topografie-gejubelt-dba162b6e735@brauner/T/


Change-Id: Ib1f5fcb707a3db43a497cf4ae663156c202d6849
Signed-off-by: Pradeep P V K <quic_pragalla@quicinc.com>
Signed-off-by: 朱陆林 <zhululin1@xiaomi.corp-partner.google.com>