staging/lustre/llite: glimpse the inode before doing fiemap

For a new inode, the i_size is 0 until a stat, which will yield
an empty fiemap result.
Fix the issue by glimpsing the size before doing fiemap.

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-on: http://review.whamcloud.com/13439
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6091


Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>