Optimize the block size used on ZFS cache devices as is already done
for data and log devices.

Reported by:	Dmitryy Makarov
Submitted by:	smh
Reviewed by:	gibbs
Approved by:	re (delphij)
MFC after:	2 weeks
Remove additional non-FreeBSD code.

Approved by:	glebius (mentor)
Approved by:	re (blanket)