Skip device if parent is a file and not a dir

[zestrada]

This PR skips creation of a device from the devtable file if the parent of that device is not a directory.

Ran into this when rebuilding a filesystem extracted from an existing image. We use a devtable to add back devices that might be missed during extraction. Sometimes you'll get stuff like /dev/mtd0 in the existing image but the default devtable has entries like /dev/mtd/0. This change allows genext2fs to be a little more robust in use cases like ours.