exfat: support batch discard of clusters when freeing clusters

If the discard mount option is enabled, the file's clusters are
discarded when the clusters are freed. Discarding clusters one by
one will significantly reduce performance. Poor performance may
cause soft lockup when lots of clusters are freed.

This commit improves performance by discarding contiguous clusters
in batches.

Measure the performance by:

  # truncate -s 80G /mnt/file
  # time rm /mnt/file

Without this commit:

  real    4m46.183s
  user    0m0.000s
  sys     0m12.863s

With this commit:

  real    0m1.661s
  user    0m0.000s
  sys     0m0.017s

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