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

Following process:
          P1                     P2
  path_lookupat
   link_path_walk
    inode_permission
     ovl_permission
       ovl_i_path_real(inode, &realpath)
         path->dentry = ovl_i_dentry_upper(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
       realinode = d_inode(realpath.dentry) // return NULL
       inode_permission(realinode)
        inode->i_sb  // NULL pointer dereference
 , will trigger an null pointer dereference at realinode:
   [  335.664979] BUG: kernel NULL pointer dereference,
                  address: 0000000000000002
   [  335.668032] CPU: 0 PID: 2592 Comm: ls Not tainted 6.3.0
   [  335.669956] RIP: 0010:inode_permission+0x33/0x2c0
   [  335.678939] Call Trace:
   [  335.679165]  <TASK>
   [  335.679371]  ovl_permission+0xde/0x320
   [  335.679723]  inode_permission+0x15e/0x2c0
   [  335.680090]  link_path_walk+0x115/0x550
   [  335.680771]  path_lookupat.isra.0+0xb2/0x200
   [  335.681170]  filename_lookup+0xda/0x240
   [  335.681922]  vfs_statx+0xa6/0x1f0
   [  335.682233]  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=217405


 Fixes: 4b7791b2 ("ovl: handle idmappings in ovl_permission()")
 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: 296173010
Link: https://lore.kernel.org/linux-unionfs/20230516-topografie-gejubelt-dba162b6e735@brauner/T/


Change-Id: If89ff32991b8075844f11efbae05edaa6465b5ee
Signed-off-by: Pradeep P V K <quic_pragalla@quicinc.com>