ovl: detect overlapping layers

Overlapping overlay layers are not supported and can cause unexpected
behavior, but overlayfs does not currently check or warn about these
configurations.

User is not supposed to specify the same directory for upper and
lower dirs or for different lower layers and user is not supposed to
specify directories that are descendants of each other for overlay
layers, but that is exactly what this zysbot repro did:

    https://syzkaller.appspot.com/x/repro.syz?x=12c7a94f400000

Moving layer root directories into other layers while overlayfs
is mounted could also result in unexpected behavior.

This commit places "traps" in the overlay inode hash table.
Those traps are dummy overlay inodes that are hashed by the layers
root inodes.

On mount, the hash table trap entries are used to verify that overlay
layers are not overlapping.  While at it, we also verify that overlay
layers are not overlapping with directories "in-use" by other overlay
instances as upperdir/workdir.

On lookup, the trap entries are used to verify that overlay layers
root inodes have not been moved into other layers after mount.

Some examples:

$ ./run --ov --samefs -s
...
( mkdir -p base/upper/0/u base/upper/0/w base/lower lower upper mnt
  mount -o bind base/lower lower
  mount -o bind base/upper upper
  mount -t overlay none mnt ...
        -o lowerdir=lower,upperdir=upper/0/u,workdir=upper/0/w)

$ umount mnt
$ mount -t overlay none mnt ...
        -o lowerdir=base,upperdir=upper/0/u,workdir=upper/0/w

  [   94.434900] overlayfs: overlapping upperdir path
  mount: mount overlay on mnt failed: Too many levels of symbolic links

$ mount -t overlay none mnt ...
        -o lowerdir=upper/0/u,upperdir=upper/0/u,workdir=upper/0/w

  [  151.350132] overlayfs: conflicting lowerdir path
  mount: none is already mounted or mnt busy

$ mount -t overlay none mnt ...
        -o lowerdir=lower:lower/a,upperdir=upper/0/u,workdir=upper/0/w

  [  201.205045] overlayfs: overlapping lowerdir path
  mount: mount overlay on mnt failed: Too many levels of symbolic links

$ mount -t overlay none mnt ...
        -o lowerdir=lower,upperdir=upper/0/u,workdir=upper/0/w
$ mv base/upper/0/ base/lower/
$ find mnt/0
  mnt/0
  mnt/0/w
  find: 'mnt/0/w/work': Too many levels of symbolic links
  find: 'mnt/0/u': Too many levels of symbolic links

Reported-by:  <syzbot+9c69c282adc4edd2b540@syzkaller.appspotmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>