rand: add AES random buffer generator

rand: add AES random buffer generator

This will be used by default for just buffer data generation, as I
suspect the overhead of doing it for smaller bits of data would mean
it's not really a win.

Pretty sure there are massive amounts of improvements available here,
this is just meant to serve as a proof-of-concept on the simplest
possible implementation. The grunt of this work is really doing the
detection properly and ensuring it's abstracted out nicely.

Signed-off-by: Jens Axboe <axboe@kernel.dk>