smb3: for kerberos mounts display the credential uid used

For kerberos mounts, the cruid is helpful to display in
/proc/mounts in order to tell which uid's krb5 cache we
got the ticket for and to tell in the multiuser krb5 case
which local users (uids) we have Kerberos authentic sessions
for.

Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>