mm/page_isolation: fix clang deadcode warning

When !CONFIG_VM_BUG_ON, there is warning of
clang-analyzer-deadcode.DeadStores:
Value stored to 'mt' during its initialization is never read.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Maria Yu <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Doug Berger <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Zi Yan <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/page_isolation.c

···
 				      zone->zone_start_pfn);
 
 	if (skip_isolation) {
-		int mt = get_pageblock_migratetype(pfn_to_page(isolate_pageblock));
+		int mt __maybe_unused = get_pageblock_migratetype(pfn_to_page(isolate_pageblock));
 
 		VM_BUG_ON(!is_migrate_isolate(mt));
 	} else {