Update mdadm for bionic

Mdadm version available in bionic was released in march 2018. Since it many various fixes has been added.
The most common and important of them (from IMSM site) are already reported:
https://bugs.launchpad.net/oem-priority/+bug/1868708
https://bugs.launchpad.net/ubuntu/+source/mdadm/+bug/1887640

However there is also many other bugs.
Some of them are connected with specific hardware or system configuration, others are sporadic ones which occurs in certain conditions. Here the most important fixes, but not reported:
https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=9f4218274cd4a1e1f356a1617f9a1d09960cf255
https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=69d084784de196acec8ab703cd1b379af211d624
https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=d2e11da4b7fd0453e942f43e4196dc63b3dbd708
https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=2b57e4fe041d52ae29866c93a878a11c07223cff
https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=05501181f18cdccdb0b3cec1d8cf59f0995504d7
https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=4ec389e3f0c1233f5aa2d5b4e63d96e33d2a37f0

There are also a lot of minor bugs and some features.
Maintainer is working on releasing version 4.2, but it is still in progress.

Please update mdadm to latest upstream for bionic.
I will notify if mdadm-4.2 becomes available.