NFSv4: Fix a regression against the FreeBSD server

Technically, the Linux client is allowed by the NFSv4 spec to send
3 word bitmaps as part of an OPEN request. However, this causes the
current FreeBSD server to return NFS4ERR_ATTRNOTSUPP errors.

Fix the regression by making the Linux client use a 2 word bitmap unless
doing NFSv4.2 with labeled NFS.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>