fix: :ball w_locked check, reset b_nwindows in free_all_mem() #38484

Problem:
- Small error in port of v9.1.0678, causing :ball to check w_locked for the
  wrong window.
- After #27439, free_all_mem() may not wipe out buffers that were open in more
  than one window before windows were freed.

Solution:
- Check win_locked() for wp in ex_buffer_all(), not curwin.
- Set b_nwindows to 0 in free_all_mem() before calling close_buffer().

Ref: https://github.com/neovim/neovim/pull/38473#issuecomment-4125117681
No need to block these fixes on that.

free_all_mem() change also looks like it fixed the existing "N lua references were
leaked!" warnings on the CI.

src/nvim/buffer.c

@@ -3663,7 +3663,7 @@ void ex_buffer_all(exarg_T *eap)
                : wp->w_width != Columns)
            || (had_tab > 0 && wp != firstwin))
           && !ONE_WINDOW
-          && !(win_locked(curwin) || wp->w_buffer->b_locked > 0)
+          && !(win_locked(wp) || wp->w_buffer->b_locked > 0)
           && !is_aucmd_win(wp)) {
         if (win_close(wp, false, false) == FAIL) {
           break;