security_selinux: Remember old labels

security_selinux: Remember old labels

Similarly to what I did in DAC driver, this also requires the
same SELinux label to be used for shared paths. If a path is
already in use by a domain (or domains) then and the domain we
are starting now wants to access the path it has to have the same
SELinux label. This might look too restrictive as the new label
can still guarantee access to already running domains but in
reality it is very unlikely and usually an admin mistake.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>