Use mount.SingleEntryFilter as we're only interested in a single entry.
Test case data of TestShouldUnmountRoot is modified accordingly, as
from now on:
1. `info` can't be nil;
2. the mountpoint check is not performed (as SingleEntryFilter
guarantees it to be equal to daemon.root).
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Upstream-commit: d3ebcde82aae79be8cbecab365367b17adac4b3e
Component: engine