quota: Fix potential NULL pointer dereference

Below race may cause NULL pointer dereference

P1					P2
dquot_free_inode			quota_off
					  drop_dquot_ref
					   remove_dquot_ref
					   dquots = i_dquot(inode)
  dquots = i_dquot(inode)
  srcu_read_lock
  dquots[cnt]) != NULL (1)
					     dquots[type] = NULL (2)
  spin_lock(&dquots[cnt]->dq_dqb_lock) (3)
   ....

If dquot_free_inode(or other routines) checks inode's quota pointers (1)
before quota_off sets it to NULL(2) and use it (3) after that, NULL pointer
dereference will be triggered.

So let's fix it by using a temporary pointer to avoid this issue.

Signed-off-by: Wang Jianjian <wangjianjian3@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240202081852.2514092-1-wangjianjian3@huawei.com>