LU-11360 osd-ldiskfs: hold s_umount before calling sync_fs

LU-11360 osd-ldiskfs: hold s_umount before calling sync_fs

ldiskfs_sync_fs() calls dquot_writeback_dquots(), which expects
we are holding s_umount since kernel 4.10
Linux-commit: 9d1ccbe70e0b14545caad12dc73adb3605447df0

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: Icda84eebe0d6261e9e374fb2a005bb66e586bba8
Reviewed-on: https://review.whamcloud.com/33135
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>