LU-8773 llite: refactor lov_object_fiemap()

LU-8773 llite: refactor lov_object_fiemap()

* Change loff_t to u64 in lov_object_fiemap() since loff_t is a
  signed value type.
* Add fiemap_for_stripe() to get file map extent from each stripe
  device.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Ic8ac98747eb32f4be90e602a0995fad8ef211bb8
Reviewed-on: https://review.whamcloud.com/23461
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>