generator: exit early when asked to generate fsck unit for / and /usr in initrd

generator: exit early when asked to generate fsck unit for / and /usr in initrd

Let's exit early if we are invoked to generate an fsck unit for the
rootfs or /usr of the initrd itself. The "systemd-root-fsck.service" and
"systemd-usr-fsck.service" units are after all for the host file
systems, and the initrd file hierarchy is from an unpacked cpio anyway.
Hence, this semantically doesn't really make sense, so quickly exit if
we detect this case. This allows us to remove some checks further down
the codepath.