mm: add split_folio()

This wrapper removes a need to use split_huge_page(&folio->page).  Convert
two callers.

Link: https://lkml.kernel.org/r/20220902194653.1739778-5-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/huge_mm.h

@@ -444,6 +444,11 @@ static inline int split_folio_to_list(struct folio *folio,
 	return split_huge_page_to_list(&folio->page, list);
 }
 
+static inline int split_folio(struct folio *folio)
+{
+	return split_folio_to_list(folio, NULL);
+}
+
 /*
  * archs that select ARCH_WANTS_THP_SWAP but don't support THP_SWP due to
  * limitations in the implementation like arm64 MTE can override this to

mm/shmem.c

@@ -629,7 +629,7 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 			goto move_back;
 		}
 
-		ret = split_huge_page(&folio->page);
+		ret = split_folio(folio);
 		folio_unlock(folio);
 		folio_put(folio);
 

mm/truncate.c

@@ -240,7 +240,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 		folio_invalidate(folio, offset, length);
 	if (!folio_test_large(folio))
 		return true;
-	if (split_huge_page(&folio->page) == 0)
+	if (split_folio(folio) == 0)
 		return true;
 	if (folio_test_dirty(folio))
 		return false;