some minor clean up.. Always _ZERO memory so mtx_init won't panic...

use the proper macro instead of hand rolling it...

Reviewed by:	jhb (only the malloc change)
MFC after:	1 week
allow the kern.cryptodevallowsoft sysctl to enable symetric/hashes too...
This will allow us to more easily test the software versions of these
routines...

Considering that we've never had an software asymetric implmentation,
it's doubtful anyone has this enabled...