cmd/snap-confine: fix exploding homedirs bug

The combination of snap set system homedirs=/var/lib/ and a snap that causes a
mimic to be created over /var/lib is sufficient to leak over to the initial
mount namespace.

Fixes: https://warthogs.atlassian.net/browse/SNAPDENG-17066

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

cmd/snap-confine/mount-support.c

@@ -927,7 +927,8 @@ static struct sc_mount *sc_homedir_mounts(const struct sc_invocation *inv)
 	for (int i = 0; i < inv->num_homedirs; i++) {
 		debug("Adding homedir: %s", inv->homedirs[i]);
 		mounts[i].path = sc_strdup(inv->homedirs[i]);
-		mounts[i].is_bidirectional = true;
+		// Note that we are not setting bidirectional flag, so anything mounted
+		// here will not propagate to the host.
 	}
 	return mounts;
 }