https://bugzilla.redhat.com/show_bug.cgi?id=1431112

There can be nested mount points. For instance /dev/shm/blah can
be a mount point and /dev/shm too. It doesn't make much sense to
return the former path because callers preserve the latter (and
with that the former too). Therefore prune nested mount points.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>