os/FileStore: using FIEMAP_FLAGS_SYNC instead of fsync() before call
fiemap.

When call fiemap, it need sync the file. Now it used fsync() to achieve
this. But for fiemap, there is a flag FIEMAP_FLAGS_SYNC which do the
same thing like fsync().

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>