Recursive nullfs mounts and r224655

[b. f.]

Recent changes to the kernel (sys/kern/vfs_mount.c, in r224655?)
between r224550 and r224655 have broken my tinderbox setup.  It had a
tmpfs filesystem mounted at /T and a UFS filesystem mounted at /U,
and, when setting up the tinderbox, performed:

mkdir /U/u1
mkdir /U/u2
mkdir /T/t1
mount -t nullfs /T/t1 /U/u1
mkdir-p /U/u1/u3/u4
mount -t nullfs /U/u2 /U/u1/u3/u4
...

This worked at r224550 and before.  It now fails at the second nullfs
mount, with ENOENT("mount_nullfs: No such file or directory").

b.