Fix d/p/initrd-not-initramfs.patch to point to /boot/initrd.img-$kver again, and rename to initrd.img to avoid further blunders like this (Closes: #1030191)

This has been the case from the original version of this patch, for consistency with all other parts of Debian (and the normative kernel handbook: https://kernel-team.pages.debian.net/kernel-handbook/ch-packaging.html#s-arch-dep), but the confusing name meant it got refreshed to /boot/initrd-$kver

I've also separately verified that these are all the instances that we should be patching I think.

The weird name has been a consistent source of confusion from the maintainer, who updates the patch according to its filename, not according to the message in the original commit 8ffe4235

Also, the new name matches the existing naming convention