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 (cifs_kill_sb() calling close_all_cached_dirs()) 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 invalidate_all_cached_dirs(), 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 cfid_put_wq workqueue, and close_all_cached_dirs()
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 serverclose_wq workqueue; this is done separate from
dropping the dentries so that close_all_cached_dirs() 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 close_all_cached_dirs(), and
locking around the freeing of cfid->dentry.