Name a filesystem by its kernel id, never its device name (hq ADR 0325 fourth review)

An ext4 or xfs library mounted by UUID can come back from a reboot as another device
(/dev/sdb1 as /dev/sdc1); naming the filesystem by mountinfo's source read that as a
move, stripped the warranted library's entries and asked for a new Approve. A
filesystem is now named by statfs's f_fsid (from the UUID, or the ZFS dataset), its
type and its mount point. Entries left on the filesystem a folder is on now are
merged back or held, never walked away, and a TakenOn naming a mount point is not
taken.

internal/apply/walk_linux.go

@@ -538,3 +538,13 @@ func createPath(path string, stats walkStats) (int, error) {
 	unix.Close(fd)
 	return made, nil
 }
+
+// MountFsid is the kernel's id of the filesystem at point (statfs f_fsid), in hex; empty when it cannot be read.
+// A variable so a test can stand in for a filesystem it cannot mount.
+var MountFsid = func(point string) string {
+	var st unix.Statfs_t
+	if err := unix.Statfs(point, &st); err != nil {
+		return ""
+	}
+	return fmt.Sprintf("%08x%08x", uint32(st.Fsid.Val[0]), uint32(st.Fsid.Val[1]))
+}