softmmu/physmem: fix memory leak in dirty_memory_extend()

As reported by Peter, we might be leaking memory when removing the
highest RAMBlock (in the weird ram_addr_t space), and adding a new one.

We will fail to realize that we already allocated bitmaps for more
dirty memory blocks, and effectively discard the pointers to them.

Fix it by getting rid of last_ram_page() and by remembering the number
of dirty memory blocks that have been allocated already.

While at it, let's use "unsigned int" for the number of blocks, which
should be sufficient until we reach ~32 exabytes.

Looks like this leak was introduced as we switched from using a single
bitmap_zero_extend() to allocating multiple bitmaps:
bitmap_zero_extend() relies on g_renew() which should have taken care of
this.

Resolves: https://lkml.kernel.org/r/CAFEAcA-k7a+VObGAfCFNygQNfCKL=AfX6A4kScq=VSSK0peqPg@mail.gmail.com
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: 5b82b703b6 ("memory: RCU ram_list.dirty_memory[] for safe RAM hotplug")
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240828090743.128647-1-david@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

include/exec/ramlist.h

@@ -50,6 +50,7 @@ typedef struct RAMList {
     /* RCU-enabled, writes protected by the ramlist lock. */
     QLIST_HEAD(, RAMBlock) blocks;
     DirtyMemoryBlocks *dirty_memory[DIRTY_MEMORY_NUM];
+    unsigned int num_dirty_blocks;
     uint32_t version;
     QLIST_HEAD(, RAMBlockNotifier) ramblock_notifiers;
 } RAMList;