CVE-2024-53176: smb: During unmount, ensure all cached dir instances drop their dentry

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

smb: During unmount, ensure all cached dir instances drop their dentry

The unmount process (cifskillsb() calling closeallcacheddirs()) can race with various cached directory operations, which ultimately results in dentries not being dropped and these kernel BUGs:

BUG: Dentry ffff88814f37e358{i=1000000000080,n=/} still in use (2) [unmount of cifs cifs] VFS: Busy inodes after unmount of cifs (cifs) ------------[ cut here ]------------ kernel BUG at fs/super.c:661!

This happens when a cfid is in the process of being cleaned up when, and has been removed from the cfids->entries list, including:

- Receiving a lease break from the server - Server reconnection triggers invalidateallcacheddirs(), which removes all the cfids from the list - The laundromat thread decides to expire an old cfid.

To solve these problems, dropping the dentry is done in queued work done in a newly-added cfidputwq workqueue, and closeallcacheddirs() flushes that workqueue after it drops all the dentries of which it's aware. This is a global workqueue (rather than scoped to a mount), but the queued work is minimal.

The final cleanup work for cleaning up a cfid is performed via work queued in the serverclosewq workqueue; this is done separate from dropping the dentries so that closeallcacheddirs() doesn't block on any server operations.

Both of these queued works expect to invoked with a cfid reference and a tcon reference to avoid those objects from being freed while the work is ongoing.

While we're here, add proper locking to closeallcacheddirs(), and locking around the freeing of cfid->dentry.