exfat: fix timing of synchronizing bitmap and inode

Commit(f55c096f exfat: do not zero the extended part) changed
the timing of synchronizing bitmap and inode in exfat_cont_expand().
The change caused xfstests generic/013 to fail if 'dirsync' or 'sync'
is enabled. So this commit restores the timing.

Fixes: f55c096f ("exfat: do not zero the extended part")
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>