cephadm: make /sys/fs/selinux empty

cephadm: make /sys/fs/selinux empty

When the following conditions are true:

  1) A host has selinux-policy-targeted,
  2) We mount the host's /sys into a privileged container,
  3) The container has SELINUXTYPE=targeted in /etc/selinux/config,
  4) The container does not have an selinux-policy-targeted package,

then SELinux-enabled applications like restorecon or DNF do not work inside
the container.

Resolve this by making /sys/fs/selinux an empty directory.

Fixes: https://tracker.ceph.com/issues/49239
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>