tool: fix tool_seek_cb build when SIZEOF_CURL_OFF_T > SIZEOF_OFF_T

- a variable was renamed, and some use of it wasn't. this fixes the
  build.

Closes #11468

src/tool_cb_see.c

@@ -67,13 +67,13 @@ int tool_seek_cb(void *userdata, curl_off_t offset, int whence)
       /* this code path doesn't support other types */
       return CURL_SEEKFUNC_FAIL;
 
-    if(LSEEK_ERROR == lseek(in->fd, 0, SEEK_SET))
+    if(LSEEK_ERROR == lseek(per->infd, 0, SEEK_SET))
       /* couldn't rewind to beginning */
       return CURL_SEEKFUNC_FAIL;
 
     while(left) {
       long step = (left > OUR_MAX_SEEK_O) ? OUR_MAX_SEEK_L : (long)left;
-      if(LSEEK_ERROR == lseek(in->fd, step, SEEK_CUR))
+      if(LSEEK_ERROR == lseek(per->infd, step, SEEK_CUR))
         /* couldn't seek forwards the desired amount */
         return CURL_SEEKFUNC_FAIL;
       left -= step;