Really zero the zero page

While switching abd_zero_buf allocation KPI I've missed the fact
that kmem_zalloc() zeroed the allocation, while kmem_cache_alloc()
does not. Add explicit bzero() after it.

I don't think it should have caused real problems, but leaking one
memory page content all over the pool is not good.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #12569