staging/lustre/ost: Ensure dirty flushed on fiemap ioctl

Data corruption is possible if cp(coreutils) uses FIEMAP to obtain
data holes in shared file, since there could be dirty cache on
other clients which hasn't been flushed back.

To ensure all the dirty on remote clients being flushed back on
fiemap ioctl, we'd acquire ldlm lock on server side for fiemap,
unless the local client (which invoke fiemap) has cached lock.

[picked osc part for upstream kernel submission]
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3219
Xyratex-bug-id: MRP-1001
Lustre-change: http://review.whamcloud.com/6127


Signed-off-by: Artem Blagodarenko <artem_blagodarenko@xyratex.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>