nfsd: don't replace page in rq_pages if it's a continuation of last page

In the Linux kernel, the following vulnerability has been resolved:

nfsd: don't replace page in rq_pages if it's a continuation of last page

The splice read calls nfsd_splice_actor to put the pages containing file
data into the svc_rqst->rq_pages array. It's possible however to get a
splice result that only has a partial page at the end, if (e.g.) the
filesystem hands back a short read that doesn't cover the whole page.

nfsd_splice_actor will plop the partial page into its rq_pages array and
return. Then later, when nfsd_splice_actor is called again, the
remainder of the page may end up being filled out. At this point,
nfsd_splice_actor will put the page into the array _again_ corrupting
the reply. If this is done enough times, rq_next_page will overrun the
array and corrupt the trailing fields -- the rq_respages and
rq_next_page pointers themselves.

If we've already added the page to the array in the last pass, don't add
it to the array a second time when dealing with a splice continuation.
This was originally handled properly in nfsd_splice_actor, but commit
91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()") removed the check
for it.