selinux: don't relabel /run/initramfs

/run/initramfs usually contains the initrd so that we can jump back into
it on shutdown. It's usually relatively large and static data, hence we
should avoid relabelling of it. On my netbook this saves 6s. (6.6s
needed for relabelling /dev and /run goes down to 600ms -- still way too
much, but much better).