smb: client: Avoid race in open_cached_dir with lease breaks

A pre-existing valid cfid returned from find_or_create_cached_dir might
race with a lease break, meaning open_cached_dir doesn't consider it
valid, and thinks it's newly-constructed. This leaks a dentry reference
if the allocation occurs before the queued lease break work runs.

Avoid the race by extending holding the cfid_list_lock across
find_or_create_cached_dir and when the result is checked.

Cc: stable@vger.kernel.org
Reviewed-by: Henrique Carvalho <henrique.carvalho@suse.com>
Signed-off-by: Paul Aurich <paul@darkrain42.org>
Signed-off-by: Steve French <stfrench@microsoft.com>