nfsd/nfsd3_proc_readdir: fix buffer count and page pointers

After this commit
  f875a79 nfsd: allow nfsv3 readdir request to be larger.
nfsv3 readdir request size can be larger than PAGE_SIZE. So if the
directory been read is large enough, we can use multiple pages
in rq_respages. Update buffer count and page pointers like we do
in readdirplus to make this happen.

Now listing a directory within 3000 files will panic because we
are counting in a wrong way and would write on random page.

Fixes: f875a79 "nfsd: allow nfsv3 readdir request to be larger"
Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs3xdr.c

@@ -573,6 +573,7 @@ int
 nfs3svc_decode_readdirargs(struct svc_rqst *rqstp, __be32 *p)
 {
 	struct nfsd3_readdirargs *args = rqstp->rq_argp;
+	int len;
 	u32 max_blocksize = svc_max_payload(rqstp);
 
 	p = decode_fh(p, &args->fh);
@@ -582,8 +583,14 @@ nfs3svc_decode_readdirargs(struct svc_rqst *rqstp, __be32 *p)
 	args->verf   = p; p += 2;
 	args->dircount = ~0;
 	args->count  = ntohl(*p++);
-	args->count  = min_t(u32, args->count, max_blocksize);
-	args->buffer = page_address(*(rqstp->rq_next_page++));
+	len = args->count  = min_t(u32, args->count, max_blocksize);
+
+	while (len > 0) {
+		struct page *p = *(rqstp->rq_next_page++);
+		if (!args->buffer)
+			args->buffer = page_address(p);
+		len -= PAGE_SIZE;
+	}
 
 	return xdr_argsize_check(rqstp, p);
 }