Workaround issue of Linux vdev_disk.c, (#16678)

in some cases not linearizing buffers with disk sector crossing a
page boundary. It is fine for hardware, but somehow required by LUKS.
It is not typical for ZFS to produce such buffers, but it may happen
if 6KB block is compressed to 4KB, while still having 2KB alignment.
Banning the 6KB buffers helps vdevs with ashifh=12.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>