exfat: update file system parameter handling

Al Viro recently reworked the way file system parameters are handled
Update super.c to work with it in linux-next 20200203.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>