core: mount-setup: handle non-existing mountpoints gracefully

Commit e792e89 ("path-util: don't eat up ENOENT in
path_is_mount_point()") changed path_is_mount_point() so it doesn't hide
-ENOENT from its caller. This causes all boots to fail early in case
any of the mount points does not exist (for instance, when kdbus isn't
loaded, /sys/fs/kdbus is missing).

Fix this by returning 0 from mount_one() if path_is_mount_point()
returned -ENOENT.

src/core/mount-setup.c

@@ -157,6 +157,9 @@ static int mount_one(const MountPoint *p, bool relabel) {
                 label_fix(p->where, true, true);
 
         r = path_is_mount_point(p->where, true);
+        if (r == -ENOENT)
+                return 0;
+
         if (r < 0)
                 return r;