nfsd: drop the ncf_cb_bmap field

This is always the same value, and in a later patch we're going to need
to set bits in WORD2. We can simplify this code and save a little space
in the delegation too. Just hardcode the bitmap in the callback encode
function.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>