Be consistent about _KERNEL vs _HARDKERNEL in zfs.

As it happens, on x86 both _HARDKERNEL and _KERNEL get defined; see
the conditional in sys/rump/Makefile.rump that _refrains_ from
defining _RUMPKERNEL on x86.

So the only version of this code that has been tested is the one with
all of it included.  But on, e.g., aarch64, we do not get _HARDKERNEL
here, and the code fails to build because some things use the field
struct spa::spa_deadman_cycid under _KERNEL when it is declared only
under _HARDKERNEL.

If there's a reason _not_ to use this in rump -- and it's not obvious
to me why -- then all access to the field needs to agree to use
_HARDKERNEL.

(riastradh)