cifs: fetch credentials out of keyring for non-krb5 auth multiuser mounts

Fix up multiuser mounts to set the secType and set the username and
password from the key payload in the vol info for non-krb5 auth types.

Look for a key of type "secret" with a description of
"cifs:a:<server address>" or "cifs:d:<domainname>". If that's found,
then scrape the username and password out of the key payload and use
that to create a new user session.

Finally, don't have the code enforce krb5 auth on multiuser mounts,
but do require a kernel with keys support.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>