core/credential,mount: re-read /proc/self/mountinfo before invoking umount command

core/credential,mount: re-read /proc/self/mountinfo before invoking umount command

When a unit has credentials, stopping the service unmounts the credentials
directory. On shutdown, stopping the service and the corresponding mount
unit may be done mostly simultaneously, and if we invoke umount command soon
after umount() being called on stopping the service, the mount unit will
fail.

This makes Mount.invalidated_state flag set when umount() is called for a path,
and re-read /proc/self/mouninfo before invoking umount command if the flag is set.

Fixes #25527.
Replaces #26959.