Mdmonitor: Omit non-md devices

Mdmonitor: Omit non-md devices

Fix segfault commit [1] introduced check whether given device is
mddevice, but it happend to terminate Mdmonitor if at least one of given
devices didn't fulfill that condition. In result Mdmonitor service was
no longer started on boot (with --scan option) when config contained some
non-existent array entry.

This commit introduces ommiting non-md devices so scan option can still
be used when config is wrong and allow Mdmonitor service to run on boot.

Giving a list of devices to monitor containing non-existing or
non-md devices will result in monitoring only confirmed mddevices.

[1] https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/commit/?id=e702f392959d1c2ad2089e595b52235ed97b4e18

Signed-off-by: Lukasz Florczak <lukasz.florczak@linux.intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>