hostfs: Convert hostfs_read_folio() to use a folio

Remove the use of page APIs, including setting/clearing the error flag
which is never checked on hostfs folios.  This does not include support
for large folios as kmap_local_folio() maps only one page at a time.

Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: linux-um@lists.infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20240530202110.2653630-8-willy@infradead.orgSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/hostfs/hostfs_kern.c

@@ -432,31 +432,20 @@ static int hostfs_writepage(struct page *page, struct writeback_control *wbc)
 
 static int hostfs_read_folio(struct file *file, struct folio *folio)
 {
-	struct page *page = &folio->page;
 	char *buffer;
-	loff_t start = page_offset(page);
+	loff_t start = folio_pos(folio);
 	int bytes_read, ret = 0;
 
-	buffer = kmap_local_page(page);
+	buffer = kmap_local_folio(folio, 0);
 	bytes_read = read_file(FILE_HOSTFS_I(file)->fd, &start, buffer,
 			PAGE_SIZE);
-	if (bytes_read < 0) {
-		ClearPageUptodate(page);
-		SetPageError(page);
+	if (bytes_read < 0)
 		ret = bytes_read;
-		goto out;
-	}
-
-	memset(buffer + bytes_read, 0, PAGE_SIZE - bytes_read);
-
-	ClearPageError(page);
-	SetPageUptodate(page);
-
- out:
-	flush_dcache_page(page);
+	else
+		buffer = folio_zero_tail(folio, bytes_read, buffer);
 	kunmap_local(buffer);
-	unlock_page(page);
 
+	folio_end_read(folio, ret == 0);
 	return ret;
 }