MDEV-32552 Write-ahead logging is broken for freed pages

buf_page_free(): Flag the freed page as modified if it is found in
the buffer pool.

buf_flush_page(): If the page has been freed, ensure that the log
for it has been durably written, before removing the page
from buf_pool.flush_list.

FindBlockX: Find also MTR_MEMO_PAGE_X_MODIFY in order to avoid an
occasional failure of innodb.innodb_defrag_concurrent, which involves
freeing and reallocating pages in the same mini-transaction.

This fixes a regression that was introduced in
commit a35b4ae8 (MDEV-15528).

This logic was tested by commenting out the $shutdown_timeout line
from a test and running the following:

./mtr --rr innodb.scrub
rr replay var/log/mysqld.1.rr/mariadbd-0

A breakpoint in the modified buf_flush_page() was hit, and the
FIL_PAGE_LSN of that page had been last modified during the
mtr_t::commit() of a mini-transaction where buf_page_free()
had been executed on that page.

storage/innobase/buf/buf0buf.cc

@@ -2324,6 +2324,7 @@ void buf_page_free(fil_space_t *space, uint32_t page, mtr_t *mtr,
       mtr->memo_push(block, MTR_MEMO_PAGE_X_FIX);
       rw_lock_x_lock_inline(&block->lock, 0, file, line);
       buf_block_dbg_add_level(block, SYNC_NO_ORDER_CHECK);
+      mtr->set_modified(*block);
 
 #ifdef BTR_CUR_HASH_ADAPT
       if (block->index)

storage/innobase/buf/buf0flu.cc

@@ -875,7 +875,18 @@ static bool buf_flush_page(buf_page_t *bpage, bool lru, fil_space_t *space)
   page_t *frame= bpage->zip.data;
 
   if (status == buf_page_t::FREED)
+  {
+    if (UNIV_LIKELY(space->purpose == FIL_TYPE_TABLESPACE))
+    {
+      const lsn_t lsn= mach_read_from_8(my_assume_aligned<8>
+                                        (FIL_PAGE_LSN + (frame ? frame
+                                                         : block->frame)));
+      ut_ad(lsn >= oldest_modification);
+      if (lsn > log_sys.get_flushed_lsn())
+        log_write_up_to(lsn, true);
+    }
     buf_pool.release_freed_page(&block->page);
+  }
   else
   {
     space->reacquire();

storage/innobase/mtr/mtr0mtr.cc

@@ -1027,7 +1027,8 @@ struct FindBlockX
   /** @return whether the block was not found x-latched */
   bool operator()(const mtr_memo_slot_t *slot) const
   {
-    return slot->object != &block || slot->type != MTR_MEMO_PAGE_X_FIX;
+    return slot->object != &block ||
+      (slot->type | MTR_MEMO_MODIFY) != MTR_MEMO_PAGE_X_MODIFY;
   }
 };