Ignore pool ashift property during vdev attachment

Ashift can be set for a vdev only during its creation, and the
top-level vdev does not change when a vdev is attached or replaced.
The ashift property should not be used during attachment, as it
does not allow attaching/replacing a vdev if the pool's ashift
property is increased after the existing vdev was created. Instead,
we should be able to attach the vdev if the attached vdev can
satisfy the ashift requirement with its parent.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #15061