fuse: fix retrieve length

In some cases fuse_retrieve() would return a short byte count if offset was
non-zero.  The data returned was correct, though.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: stable@vger.kernel.org

fs/fuse/dev.c

@@ -1576,6 +1576,7 @@ static int fuse_retrieve(struct fuse_conn *fc, struct inode *inode,
 		req->pages[req->num_pages] = page;
 		req->num_pages++;
 
+		offset = 0;
 		num -= this_num;
 		total_len += this_num;
 		index++;