shared/clean-ipc: improve error message a bit

shared/clean-ipc: improve error message a bit

Failed to enter shared memory directory multipath: Permission denied
→
Failed to enter shared memory directory /dev/shm/multipath: Permission denied

When looking at nested directories, we will print only the final two elements
of the path. That is still more useful than just the last component of the
path. To print the full path, we'd have to allocate the string, and since the
error occurs so very rarely, I think the current best-effort approach is
enough.