mm/page_alloc: drop the unnecessary pfn_valid() for start pfn

mm/page_alloc: drop the unnecessary pfn_valid() for start pfn

__pageblock_pfn_to_page() currently performs both pfn_valid check and
pfn_to_online_page().  The former one is redundant because the latter is a
stronger check.  Drop pfn_valid().

Link: https://lkml.kernel.org/r/c3868b58c6714c09a43440d7d02c7b4eed6e03f6.1682342634.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>