NFSD: Change the way the expected length of a fattr4 is checked

Because the fattr4 is now managed in an xdr_stream, all that is
needed is to store the initial position of the stream before
decoding the attribute list. Then the actual length of the list
is computed using the final stream position, after decoding is
complete.

No behavior change is expected.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>