mm/page_alloc: simplify update of pgdat in wake_all_kswapds

mm/page_alloc: simplify update of pgdat in wake_all_kswapds

There is no need to update last_pgdat for each zone, only update
last_pgdat when iterating the first zone of a node.

Link: https://lkml.kernel.org/r/20220322115635.2708989-1-chenwandun@huawei.com
Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>