use a straight buffer instead of an iov w/ 1 segment... The aesni
driver when it hits a mbuf/iov buffer, it mallocs and copies the data
for processing.. This improves perf by ~8-10% on my machine...

I have thoughts of fixing AES-NI so that it can better handle segmented
buffers, which should help improve IPSEC performance, but that is for
the future...