umount: add info that --all-targets works with mtab symlink only

Reported-by: Ruediger Meier <sweet_f_a@gmx.de>
Signed-off-by: Karel Zak <kzak@redhat.com>
tests: skip umount-alltargets on regular mtab

Signed-off-by: Karel Zak <kzak@redhat.com>