mm/page_alloc: add lockdep assertion for pageblock type change

mm/page_alloc: add lockdep assertion for pageblock type change

Since the migratetype hygiene patches [0], the locking here is
a bit more formalised, so write it down with an assert.

[0] https://lore.kernel.org/lkml/20240320180429.678181-3-hannes@cmpxchg.org/T/

Link: https://lkml.kernel.org/r/20250227-pageblock-lockdep-v1-1-3701efb331bb@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>