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>
Is it better to put align logic codes to GenericFileStoreBackend.cc? FileStore::_do_sparse_copy_range also use fiemap which is expected get correct result too.