libmount: don't ignore root fs on umount -a

The release v2.22 (the new umount) introduces a regression when root
fs is excluded from umount --all. There is not reason for this
exception. The libmount should be smart enough to disable mtab update
after rootfs umount.

Reported-by: Bruce Dubbs <bruce.dubbs@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>