NFS: Update NFS4_BITMAP_SIZE

v4.1/v4.2 have define attributes at word2, nfs client also support
security label now.

v3, same as v2.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>