NFSD: Use a bitmask loop to encode FATTR4 results

The fattr4 encoder is now structured like the COMPOUND op encoder:
one function for each individual attribute, called by bit number.
Benefits include:

- The individual attributes are now guaranteed to be encoded in
  bitmask order into the send buffer

- There can be no unwanted side effects between attribute encoders

- The code now clearly documents which attributes are /not/
  implemented on this server
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>