Restrict reading old_addr and new_addr to root only

As mentioned on the live-patching mailing list, it is probably a good idea to restrict reading `old_addr` and `new_addr` to root only, to prevent unprivileged users from gaining information about the kernel's address space.

That being said, we could also take this time to double check current permissions in sysfs. Should `checksum` and `enabled` be readable by root only as well? Should the perms on the kpatch directory in sysfs be more stringent?