staging: exfat: make staging/exfat and fs/exfat mutually exclusive

staging: exfat: make staging/exfat and fs/exfat mutually exclusive

Make staging/exfat and fs/exfat mutually exclusive to select the one
between two same filesystem.

Suggested-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>