NFSv4.1: update nfs4_fattr_bitmap_maxsz

Attribute IDs assigned in RFC 5661 now require three bitmaps.
Fixes hitting a BUG_ON in xdr_shrink_bufhead when getting ACLs.

Signed-off-by: Andy Adamson <andros@netapp.com>
Cc:stable@kernel.org [2.6.39]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>