Reinstate the old zpool read label logic as a fallback

In case of AIO failure, we should probably fallback to the old
behavior and still work.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Alan Somers <asomers@gmail.com>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12032
Closes #12040