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

(cherry picked from commit 82601b02ea0b96db084c04758abf7f43d57a7098)

Ticket: #3626