dm crypt: add middle-endian variant of plain64 IV

dm crypt: add middle-endian variant of plain64 IV

The big-endian IV (plain64be) backend implementation has bugs and
ended up with a weird middle endian (left shift by 32).

Adding a new module for this.

The middle endian is this weirdness where the value is shifted
to the middle (and also wraps):

iv: 00000000000000010000000000000000
instead of:
iv: 00000000000000000000000000000001

Orabug: 28604628
Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>