selinux: Properly check TAP FD label

selinux: Properly check TAP FD label

After a4431931 the TAP FDs ale labeled with image label instead
of the process label. On the other hand, the commit was
incomplete as a few lines above, there's still old check for the
process label presence while it should be check for the image
label instead.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>