nuke: make tmpfs check only umount tmpfs

This would catch things like /tmp/cephtest/mnt.client.0, which are
used by cfuse, rbd, and kclient.
nuke: don't try unmount if we're rebooting everything anyway

This can cause issues when unmount hangs. Our automatic runs reboot
everything unconditionally, so this caused a bunch of unecessary hangs
when an fs was accidentally rendered un-unmountable.