NFS: nfs_setattr_update_inode() should clear the suid/sgid bits

When we do a 'chown' or 'chgrp', the server will clear the suid/sgid
bits. Ensure that we mirror that in nfs_setattr_update_inode().

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>