storage: mpath: Don't error on target_type=NULL
We use device-mapper to enumerate all dm devices, and filter out
the list of multipath devices by checking the target_type string
name. The code however cancels all scanning if we encounter
target_type=NULL
I don't know how to reproduce that situation, but a user was hitting
it in their setup, and inspecting the lvm2/device-mapper code shows
many places where !target_type is explicitly ignored and processing
continues on to the next device. So I think we should do the same
https://bugzilla.redhat.com/show_bug.cgi?id=
1069317
(cherry picked from commit
8f8c0feb113420625f15b7f1e17bfd719c977eeb)