CVE-2023-53083: 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 rqpages if it's a continuation of last page

The splice read calls nfsdspliceactor to put the pages containing file data into the svcrqst->rqpages 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.

nfsdspliceactor will plop the partial page into its rqpages array and return. Then later, when nfsdspliceactor is called again, the remainder of the page may end up being filled out. At this point, nfsdspliceactor will put the page into the array again corrupting the reply. If this is done enough times, rqnextpage will overrun the array and corrupt the trailing fields -- the rqrespages and rqnextpage 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 nfsdspliceactor, but commit 91e23b1c3982 ("NFSD: Clean up nfsdspliceactor()") removed the check for it.