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/[email protected]
Signed-off-by: Maria Yu <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Doug Berger <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Maria Yu

mm/page_isolation.c

@@ -330,7 +330,7 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, int flags,
 				      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 {