Avoid looping forever trying to unmount busy tmpfs

This split unmount is a bit crude, unfortunately.  A user may have set
up a bind or overlayfs mount on top of a tmpfs -- so unmounting tmpfs
first will then always result in EBUSY.

Avoid logging busy errors and fake OK.  We'll catch it later in the
second stage unmount.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/mount.c

@@ -94,10 +94,12 @@ static int unmount(const char *target)
 	int rc;
 
 	rc = umount(target);
-	if (rc)
+	if (rc && errno != EBUSY) {
 		print(2, "Failed unmounting %s, error %d: %s", target, errno, strerror(errno));
+		return rc;
+	}
 
-	return rc;
+	return 0;
 }
 
 void unmount_tmpfs(void)