smb: Initialize cfid->tcon before performing network ops

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

smb: Initialize cfid->tcon before performing network ops

Avoid leaking a tcon ref when a lease break races with opening the

cached directory. Processing the leak break might take a reference to

the tcon in cached_dir_lease_break() and then fail to release the ref in

cached_dir_offload_close, since cfid->tcon is still NULL.