LU-6091 llite: glimpse the inode before doing fiemap

LU-6091 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>
Change-Id: I072fd1f5e13c7fd127199a10b44366aed145b8e1
Reviewed-on: http://review.whamcloud.com/13280
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.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>