(followup to myself...)

>Here are the results:
>
>kernel with NFS_V2_ONLY option:
>        mount_nfs -2 ...   works
>        mount_nfs -3 ...   doesn't work, bug appears
>
>kernel without NFS_V2_ONLY option:
>        mount_nfs -2 ...   works
>        mount_nfs -3 ...   works
>

After looking at this again after some sleep, I've come to the
conclusion that the bug isn't in the kernel code, but in mount_nfs.

It generates a v3 mount even if the kernel doesn't understand v3
(NFS_V2_ONLY). One thing that then fails seems to be the getcwd()
call.

regards,
chris