Enable multi-cluster fetching for exfat_get_block.

From: Chi Zhiling <chizhiling@kylinos.cn>

This patch series significantly improves exFAT read performance
by adding multi-cluster mapping support.
The changes reduce get_block calls during sequential reads,
particularly benefiting small cluster sizes.

- Extends exfat_get_cluster() and exfat_map_cluster() to handle multiple contiguous clusters
- Adds buffer head caching for FAT table reads

Performance results show ~10% improvement for 512-byte clusters (454->511 MB/s)
and reduced get_block overhead from 10.8% to 0.02% for NO_FAT_CHAIN files.

All criticism and suggestions are welcome :)


Changes:
- Cache the last dis-continuous cluster
- Continue collect clusters after cache hit
- Some cleanup.

V1:
https://lore.kernel.org/linux-fsdevel/20251226094440.4555...
rfc:
https://lore.kernel.org/linux-fsdevel/20251118082208.1034...


Chi Zhiling (13):
  exfat: add cache option for __exfat_ent_get
  exfat: support reuse buffer head for exfat_ent_get
  exfat: improve exfat_count_num_clusters
  exfat: improve exfat_find_last_cluster
  exfat: remove the check for infinite cluster chain loop
  exfat: remove the unreachable warning for cache miss cases
  exfat: reduce the number of parameters for exfat_get_cluster()
  exfat: reuse cache to improve exfat_get_cluster
  exfat: remove handling of non-file types in exfat_map_cluster
  exfat: support multi-cluster for exfat_map_cluster
  exfat: tweak cluster cache to support zero offset
  exfat: return the start of next cache in exfat_cache_lookup
  exfat: support multi-cluster for exfat_get_cluster

 fs/exfat/cache.c    | 149 ++++++++++++++++++++++++++++----------------
 fs/exfat/exfat_fs.h |   7 +--
 fs/exfat/fatent.c   |  61 +++++++++++-------
 fs/exfat/inode.c    |  52 ++++++----------
 4 files changed, 157 insertions(+), 112 deletions(-)


base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
prerequisite-patch-id: e5f54b5b252924be0d139310a81d57eda55bead3
-- 
2.43.0