exfat: support modifying mount options via remount

Before this commit, all exfat-defined mount options could not be
modified dynamically via remount, and no error was returned.

After this commit, these three exfat-defined mount options
(discard, zero_size_dir, and errors) can be modified dynamically
via remount. While other exfat-defined mount options cannot be
modified dynamically via remount because their old settings are
cached in inodes or dentries, modifying them will be rejected with
an error.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>