nfsd: don't return high mode bits

The Linux NFS server replies among other things to a "Check access permission"
the following:

NFS:    File type = 2 (Directory)
NFS:    Mode = 040755

A netapp server replies here:
NFS:    File type = 2 (Directory)
NFS:    Mode = 0755

The RFC 1813 i read:
   fattr3

      struct fattr3 {
         ftype3     type;
         mode3      mode;
         uint32     nlink;
...
For the mode bits only the lowest 9 are defined in the RFC

As far as I can tell, knfsd has always done this, so apparently it's harmless.
Nevertheless, it appears to be wrong.

Note this is already correct in the NFSv4 case, only v2 and v3 need
fixing.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>