nfsd4: simplify server xdr->next_page use

The rpc code makes available to the NFS server an array of pages to
encod into.  The server represents its reply as an xdr buf, with the
head pointing into the first page in that array, the pages ** array
starting just after that, and the tail (if any) sharing any leftover
space in the page used by the head.

While encoding, we use xdr_stream->page_ptr to keep track of which page
we're currently using.

Currently we set xdr_stream->page_ptr to buf->pages, which makes the
head a weird exception to the rule that page_ptr always points to the
page we're currently encoding into.  So, instead set it to buf->pages -
1 (the page actually containing the head), and remove the need for a
little unintuitive logic in xdr_get_next_encode_buffer() and
xdr_truncate_encode.

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