fs_mgr: Ignore invalid DT device nodes

With treble, the vendor kernel and ramdisk are supposed to be used.
But vendors always find new and interesting ways to break things:

$ cat /proc/device-tree/firmware/android/fstab/system/dev; echo
/by-name/system_a
$

Change-Id: I826a96ebd625b52563d3cc970bf99d9b26dc093e