NFSD: Copy FATTR4 bit number definitions from RFCs

I'd like to convert nfsd4_encode_fattr() to rotate through the
attrmask using for_each_bit() instead of explicitly testing the
bitmask for each bit value. This means I need the bit numbers, as
defined in the specs, instead of our internal bitmask constants.

As a clean up, use the new spec-derived values to define the WORD#
bitmask constants.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>