mm/hugetlb: add missing annotation for gather_surplus_pages()

Sparse reports a warning at gather_surplus_pages()

warning: context imbalance in hugetlb_cow() - unexpected unlock

The root cause is the missing annotation at gather_surplus_pages()
Add the missing __must_hold(&hugetlb_lock)

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Link: http://lkml.kernel.org/r/20200214204741.94112-7-jbi.octave@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -2010,6 +2010,7 @@ struct page *alloc_huge_page_vma(struct hstate *h, struct vm_area_struct *vma,
  * of size 'delta'.
  */
 static int gather_surplus_pages(struct hstate *h, int delta)
+	__must_hold(&hugetlb_lock)
 {
 	struct list_head surplus_list;
 	struct page *page, *tmp;