nfsd: Fix up the "supattr_exclcreat" attributes

If an NFSv4 client asks us for the supattr_exclcreat, then we must
not return attributes that are unsupported by this minor version.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Fixes: 75976de6

 ("NFSD: Return word2 bitmask if setting security..,")
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>