NFSD: Rename nfsd4_encode_bitmap()

For alignment with the specification, the name of NFSD's encoder
function should match the name of the XDR type.

I've also replaced a few "naked integers" with symbolic constants
that better reflect the usage of these values.

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