NFSD: Fix reads with a non-zero offset that don't end on a page boundary

This was found when virtual machines with nfs-mounted qcow2 disks
failed to boot properly.

Reported-by: Anders Blomdell <anders.blomdell@control.lth.se>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2142132
Fixes: bfbfb61 ("nfsd_splice_actor(): handle compound pages")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: chucklever

fs/nfsd/vfs.c

@@ -871,10 +871,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	struct svc_rqst *rqstp = sd->u.data;
 	struct page *page = buf->page;	// may be a compound one
 	unsigned offset = buf->offset;
+	struct page *last_page;
 
-	page += offset / PAGE_SIZE;
-	for (int i = sd->len; i > 0; i -= PAGE_SIZE)
-		svc_rqst_replace_page(rqstp, page++);
+	last_page = page + (offset + sd->len - 1) / PAGE_SIZE;
+	for (page += offset / PAGE_SIZE; page <= last_page; page++)
+		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;