AES-NI/OCB: Use stack for temporary storage

* cipher/rijndael-aesni.c (aesni_ocb_enc, aesni_ocb_dec): Use stack
allocated 'tmpbuf' instead of output buffer as temporary storage.

This change gives (very) small improvement for performance (~0.5%) when
output buffer is unaligned.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>