nfsd4: "backfill" using write_bytes_to_xdr_buf

Normally xdr encoding proceeds in a single pass from start of a buffer
to end, but sometimes we have to write a few bytes to an earlier
position.

Use write_bytes_to_xdr_buf for these cases rather than saving a pointer
to write to.  We plan to rewrite xdr_reserve_space to handle encoding
across page boundaries using a scratch buffer, and don't want to risk
writing to a pointer that was contained in a scratch buffer.

Also it will no longer be safe to calculate lengths by subtracting two
pointers, so use xdr_buf offsets instead.

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