Modify "4958 zdb trips assert on pools with ashift >= 0xe".

Unlike Illumos FreeBSD has concept of logical ashift, that specifies
really minimal vdev block size that can be accessed. This knowledge
allows properly pad physical I/O and correctly assert its alignment.

This change fixes L2ARC write errors when device has logical sector
size above 512 bytes.

MFC after: 1 month