BACKPORT: exfat: improve write performance when dirsync enabled

Degradation of write speed caused by frequent disk access for cluster
bitmap update on every cluster allocation could be improved by
selective syncing bitmap buffer. Change to flush bitmap buffer only
for the directory related operations.

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>
Change-Id: I660931d6da488880337a33dd03b48cb0be0bb26c
Signed-off-by: Howard Chen <howardsoc@google.com>
(cherry picked from commit 23befe49)
Bug: 233712676
(cherry picked from commit 74769685)