f2fs: avoid to create an empty string as the extension_list

When creating a file, we need to set the temperature based on
extension_list. If the empty string is a valid extension_list,
the is_extension_exist will always returns true,
which affects the separation of hot and cold.
Signed-off-by: Wang Xiaojun <wangxiaojun11@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/sysfs.c

@@ -351,7 +351,7 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 			set = false;
 		}
 
-		if (strlen(name) >= F2FS_EXTENSION_LEN)
+		if (!strlen(name) || strlen(name) >= F2FS_EXTENSION_LEN)
 			return -EINVAL;
 
 		down_write(&sbi->sb_lock);