mm/page_alloc: warn on nr_reserved_highatomic underflow

As documented in the comment this underflow should not happen.  The
locking has indeed changed here since the comment was written, see the
migratetype hygiene patches[0].  However, those changes made the locking
_safer_, so the underflow _really_ shouldn't happen now.  So upgrade the
comment to a warning.

[0] https://lore.kernel.org/all/20240320180429.678181-7-hannes@cmpxchg.org/T/#m3da87e6cc3348a4640aa298137bc9f8f61b76c84

Link: https://lkml.kernel.org/r/20250225-warn-underflow-v1-1-3dc542941d3a@google.com


Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>