exfat: improve performance of exfat_free_cluster when using dirsync mount option

There are stressful update of cluster allocation bitmap when using
dirsync mount option which is doing sync buffer on every cluster bit
clearing. This could result in performance degradation when deleting
big size file.
Fix to update only when the bitmap buffer index is changed would make
less disk access, improving performance especially for truncate operation.

Testing with Samsung 256GB sdcard, mounted with dirsync option
(mount -t exfat /dev/block/mmcblk0p1 /temp/mount -o dirsync)

Remove 4GB file, blktrace result.
[Before] : 39 secs.
Total (blktrace):
 Reads Queued:      0,        0KiB   Writes Queued:      32775,    16387KiB
 Read Dispatches:   0,        0KiB   Write Dispatches:   32775,    16387KiB
 Reads Requeued:    0                Writes Requeued:        0
 Reads Completed:   0,        0KiB   Writes Completed:   32775,    16387KiB
 Read Merges:       0,        0KiB   Write Merges:           0,        0KiB
 IO unplugs:        2                Timer unplugs:          0

[After] : 1 sec.
Total (blktrace):
 Reads Queued:      0,        0KiB   Writes Queued:         13,        6KiB
 Read Dispatches:   0,        0KiB   Write Dispatches:      13,        6KiB
 Reads Requeued:    0                Writes Requeued:        0
 Reads Completed:   0,        0KiB   Writes Completed:      13,        6KiB
 Read Merges:       0,        0KiB   Write Merges:           0,        0KiB
 IO unplugs:        1                Timer unplugs:          0

Signed-off-by: Hyeongseok Kim <hyeongseok@gmail.com>
Acked-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>