Fix bug #7589 - ntlm_auth fails to use cached credentials.

Fix bug #7589 - ntlm_auth fails to use cached credentials.

In handling the WINBINDD_PAM_AUTH message winbindd canonicalizes a *copy*
of the mapped username, but fails to canonicalize the actual username
sent to the backend domain process. When "winbind default domain"
is set this can lead to credentials being cached with an index of
user: user, not DOMAIN\user. All other code paths that use
canonicalize_username() (WINBINDD_PAM_CHAUTHTOK, WINBINDD_PAM_LOGOFF)
correctly canonicalize the data sent to the backend. All calls
the can cause credentials to be looked up (PAM_CHAUTHTOK etc.)
correctly call canonicalize_username() to create the credential
lookup key.

Jeremy.