cifs: allow changing password during remount

There are cases where a session is disconnected and password has changed
on the server (or expired) for this user and this currently can not
be fixed without unmount and mounting again.  This patch allows
remount to change the password (for the non Kerberos case, Kerberos
ticket refresh is handled differently) when the session is disconnected
and the user can not reconnect due to still using old password.

Future patches should also allow us to setup the keyring (cifscreds)
to have an "alternate password" so we would be able to change
the password before the session drops (without the risk of races
between when the password changes and the disconnect occurs -
ie cases where the old password is still needed because the new
password has not fully rolled out to all servers yet).

Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>