nfsd_splice_actor(): handle compound pages

pipe_buffer might refer to a compound page (and contain more than a PAGE_SIZE
worth of data).  Theoretically it had been possible since way back, but
nfsd_splice_actor() hadn't run into that until copy_page_to_iter() change.
Fortunately, the only thing that changes for compound pages is that we
need to stuff each relevant subpage in and convert the offset into offset
in the first subpage.

Acked-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Benjamin Coddington <bcodding@redhat.com>
Fixes: f0f6b614f8 "copy_page_to_iter(): don't split high-order page in case of ITER_PIPE"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nfsd/vfs.c

@@ -846,10 +846,14 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 		  struct splice_desc *sd)
 {
 	struct svc_rqst *rqstp = sd->u.data;
+	struct page *page = buf->page;	// may be a compound one
+	unsigned offset = buf->offset;
 
-	svc_rqst_replace_page(rqstp, buf->page);
-	if (rqstp->rq_res.page_len == 0)
-		rqstp->rq_res.page_base = buf->offset;
+	page += offset / PAGE_SIZE;
+	for (int i = sd->len; i > 0; i -= PAGE_SIZE)
+		svc_rqst_replace_page(rqstp, page++);
+	if (rqstp->rq_res.page_len == 0)	// first call
+		rqstp->rq_res.page_base = offset % PAGE_SIZE;
 	rqstp->rq_res.page_len += sd->len;
 	return sd->len;
 }