nfsd4: simplify nfsd4_encode_fattr interface slightly

It seems slightly simpler to make nfsd4_encode_fattr rather than its
callers responsible for advancing the write pointer on success.

(Also: the count == 0 check in the verify case looks superfluous.
Running out of buffer space is really the only reason fattr encoding
should fail with eresource.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>