btrfs: add set_folio_extent_mapped() helper

Turn set_page_extent_mapped() into a wrapper around this version.
Saves a call to compound_head() for callers who already have a folio
and removes a couple of users of page->mapping.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -930,17 +930,21 @@ static int attach_extent_buffer_folio(struct extent_buffer *eb,
 
 int set_page_extent_mapped(struct page *page)
 {
-	struct folio *folio = page_folio(page);
+	return set_folio_extent_mapped(page_folio(page));
+}
+
+int set_folio_extent_mapped(struct folio *folio)
+{
 	struct btrfs_fs_info *fs_info;
 
-	ASSERT(page->mapping);
+	ASSERT(folio->mapping);
 
 	if (folio_test_private(folio))
 		return 0;
 
-	fs_info = btrfs_sb(page->mapping->host->i_sb);
+	fs_info = btrfs_sb(folio->mapping->host->i_sb);
 
-	if (btrfs_is_subpage(fs_info, page->mapping))
+	if (btrfs_is_subpage(fs_info, folio->mapping))
 		return btrfs_attach_subpage(fs_info, folio, BTRFS_SUBPAGE_DATA);
 
 	folio_attach_private(folio, (void *)EXTENT_FOLIO_PRIVATE);

fs/btrfs/extent_io.h

@@ -229,6 +229,7 @@ int btree_write_cache_pages(struct address_space *mapping,
 void extent_readahead(struct readahead_control *rac);
 int extent_fiemap(struct btrfs_inode *inode, struct fiemap_extent_info *fieinfo,
 		  u64 start, u64 len);
+int set_folio_extent_mapped(struct folio *folio);
 int set_page_extent_mapped(struct page *page);
 void clear_page_extent_mapped(struct page *page);