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.