Avoid memory allocations when the given address is already 16 bytes aligned.
Such an address can be used directly in padlock's AES.
This improves speed of geli(8) significantly:

1. sysctl kern.geom.zero.clear=0
2. geli onetime -s 4096 gzero
3. dd if=/dev/gzero.eli of=/dev/null bs=1m count=1000

Before: 113MB/s
After: 203MB/s

BTW. If sector size is set to 128kB, I can read at 276MB/s :)