mount: don't claim a device is gone from /proc/self/mountinfo before it is gone from *all* lines

Devices might be referenced by multiple mount points in
/proc/self/mountinfo, hence we should consider them unmounted only after
they disappeared from all lines, not just from one.

http://lists.freedesktop.org/archives/systemd-devel/2015-May/032026.html
(cherry picked from commit fcd8b266)