mm/hugetlb: use list_splice to merge two list at once

Instead of add allocated file_region one by one to region_cache, we could
use list_splice to merge two list at once.

Also we know the number of entries in the list, increase the number
directly.
Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: https://lkml.kernel.org/r/20200831022351.20916-4-richard.weiyang@linux.alibaba.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -443,11 +443,8 @@ static int allocate_file_region_entries(struct resv_map *resv,
 
 		spin_lock(&resv->lock);
 
-		list_for_each_entry_safe(rg, trg, &allocated_regions, link) {
-			list_del(&rg->link);
-			list_add(&rg->link, &resv->region_cache);
-			resv->region_cache_count++;
-		}
+		list_splice(&allocated_regions, &resv->region_cache);
+		resv->region_cache_count += to_allocate;
 	}
 
 	return 0;