tmpfs: fix race on handling dquot rbtree

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

tmpfs: fix race on handling dquot rbtree

A syzkaller reproducer found a race while attempting to remove dquot information from the rb tree.

Fetching the rb_tree root node must also be protected by the dqopt->dqio_sem, otherwise, giving the right timing, shmem_release_dquot() will trigger a warning because it couldn't find a node in the tree, when the real reason was the root node changing before the search starts:

Thread 1 Thread 2 - shmem_release_dquot() - shmem_{acquire,release}_dquot()

• fetch ROOT - Fetch ROOT

• acquire dqio_sem
• wait dqio_sem

• do something, triger a tree rebalance
• release dqio_sem

- acquire dqio_sem - start searching for the node, but from the wrong location, missing the node, and triggering a warning.