MDEV-22139 fseg_free_page_low() fails to write FREE_PAGE record, breaking recovery

This bug was introduced by MDEV-15528
commit a35b4ae8.

In the case that I analyzed, we failed to apply
an EXTENDED,INSERT_REUSE_REDUNDANT redo log record whose
preceding record points to unallocated area after PAGE_HEAP_TOP.

Had we properly written the FREE_PAGE record for the page,
recovery would have processed it, because during the checkpoint,
the log had been completely written past the LSN of the missed write.

fseg_free_page_low(): Always invoke mtr_t::free().
The other call is in fsp_free_page().

storage/innobase/fsp/fsp0fsp.cc

@@ -2605,6 +2605,8 @@ fseg_free_page_low(
 			    xdes, xoffset, mtr);
 		fsp_free_extent(space, offset, mtr);
 	}
+
+	mtr->free(page_id_t(space->id, offset));
 }
 
 #ifndef BTR_CUR_HASH_ADAPT