hostfs: Fix the NULL vs IS_ERR() bug for __filemap_get_folio()

The __filemap_get_folio() function returns error pointers.
It never returns NULL. So use IS_ERR() to check it.

Fixes: 1da8661 ("fs: Convert aops->write_begin to take a folio")
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>

Author: ZhangPeng

fs/hostfs/hostfs_kern.c

@@ -472,8 +472,8 @@ static int hostfs_write_begin(struct file *file, struct address_space *mapping,
 
 	*foliop = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
 			mapping_gfp_mask(mapping));
-	if (!*foliop)
-		return -ENOMEM;
+	if (IS_ERR(*foliop))
+		return PTR_ERR(*foliop);
 	return 0;
 }