Don't use conv=sparse when we are using small blocks.  The
boot file system is mostly full and therefore the optimization
would be less useful, and skipping the zero area may leave
some stale data, when they are covering metadata, they could
confuse the file system.

A better solution would probably be to use newfs+restore,
which writes only to the areas that matters.  However since
we are going to ZFS so just take a conservative approach
in this branch now.

Ticket: #5808