update TODO
fstab-generator: rename x-initrd-rootfs.mount to x-initrd.rootfs

This changes the fstab mount option x-initrd-rootfs.mount to
x-initrd.rootfs, in order to only use a single namespace "x-initrd." for
all mount options of the initrd.
fstab-generator: add missing OOM check
fstab-generator: fix minor memory leak on error path