exfat: fix s_maxbytes

With fallocate support, xfstest unit generic/213 fails with

   QA output created by 213
   We should get: fallocate: No space left on device
   Strangely, xfs_io sometimes says "Success" when something went wrong
  -fallocate: No space left on device
  +fallocate: File too large

because sb->s_maxbytes is set to the volume size.

To be in line with other non-extent-based filesystems, set to max volume
size possible with the cluster size of the volume.

Signed-off-by: David Timber <dxdt@dev.snart.me>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>