SMB311: Fix reconnect

The preauth hash was not being recalculated properly on reconnect
of SMB3.11 dialect mounts (which caused access denied repeatedly
on auto-reconnect).

Fixes: 8bd68c6e ("CIFS: implement v3.11 preauth integrity")

Signed-off-by: Steve French <smfrench@gmail.com>
CC: Stable <stable@vger.kernel.org>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>