ReplicatedPG: For MAPEXT, using bl.length() instead of op.extent.length as num_rd_kb.

For fiemap, it don't real read the data(offset,length). It only fetech the infos of
extents. So don't use length as num_rd_kb.
Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
FileStore: If FileStoreBackend don't support or disable fiemap,
it should return -EOPNOTSUPP rather than send (offset, length) to user.

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