cifs: avoid extra calls to strlen() in cifs_get_spnego_key()

Since 'snprintf()' returns the number of characters emitted, an
output position may be advanced with this return value rather
than using an explicit calls to 'strlen()'. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Steve French <stfrench@microsoft.com>