FreeBSD: Use ashift in vdev_check_boot_reserve()

We should not hardcode 512-byte read size when checking for loader
in the boot area before RAIDZ expansion. Disk might be unable to
handle that I/O as is, and the code zio_vdev_io_start() handling
the padding asserts doing it only for top-level vdev.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16942