cryptodisk: Fix cipher IV mode "plain64" always being set as "plain"

cryptodisk: Fix cipher IV mode "plain64" always being set as "plain"

When setting cipher IV mode, detection is done by prefix matching the
cipher IV mode part of the cipher mode string. Since "plain" matches
"plain64", we must check for "plain64" first. Otherwise, "plain64" will
be detected as "plain".

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>