exfat: clear the volume dirty flag only when remounting read-only

exfat_reconfigure() does not hold s_lock while calling
sync_filesystem(). Therefore, the filesystem can become dirty again
between the sync and clearing the volume dirty flag, leaving a dirty
filesystem with a clean volume flag.

Holding s_lock across sync_filesystem() is not an option because it can
lead to a deadlock: writeback takes s_lock in exfat_write_inode().
Instead, clear the volume dirty flag only when remounting the filesystem
read-only, where no writer can modify the filesystem after the sync.

For a normal read-only remount, reconfigure_super() calls
sb_prepare_remount_readonly() before ->reconfigure(), which returns
-EBUSY if any writer is still active. Afterwards
sb_start_ro_state_change() sets sb->s_readonly_remount, so
mnt_get_write_access() fails with -EROFS and no new writer can start
until the reconfiguration finishes. Therefore no writer can race with
the sync and the clearing.

Forced remounts are different: with SB_FORCE, reconfigure_super() skips
sb_prepare_remount_readonly(), so the writers active at that moment are
not blocked and can still dirty the filesystem after the sync. Do not
clear the volume dirty flag in this case.

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

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