2.4.20 autofs: non-toplevel mounts broken?

[Bob Proulx <bob@proulx.com>]

After upgrading to 2.4.20 I seem to have lost the ability to autofs
mount non-toplevel mounts.  This worked in 2.4.18.  Does not work in
2.4.20.  Has anyone else seen this?

Here is the situation.  An NFS server machine exports /mnt/a, /mnt/b,
etc. but does not export the root filesystem.  With 2.4.18 and autofs
these were automatically mounted fine.  But after an upgrade to 2.4.20
from woody-proposed-updates this now fails.  The /var/log/syslog shows
this message:

On server, /etc/exports:
  /mnt/a *(rw)

  Jul 29 22:43:52 misery automount[26757]: mount(nfs): calling mkdir_path /net/torment/mnt/a
  Jul 29 22:43:52 misery automount[26757]: mount(nfs): mkdir_path torment/mnt/a failed: Operation not permitted

It is still possible to autofs nfs mount systems which do export the
full hierarchy of filesystems from the root down.

On server, /etc/exports:
  / *(rw)
  /mnt/a *(rw)

If I return to the previous kernel then the mkdir_path does not log an
"Operation not permitted" message and everything succeeds.

Something really strange to my thinking is that the automount daemon
from autofs did not change.  Only the kernel was updated.  The autofs
side should not have changed.  But since it is now not working I can
only assume this is an interaction with the kernel which is causing
this problem.

Any clues?

Thanks
Bob