ReplicatedPG: Don't use the return value of getattr_maybe_cache as attr length.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
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: Jianpeng Ma <jianpeng.ma@intel.com>
Merge pull request #2530 from majianpeng/fiemap

Fiemap

Reviewed-by: Sage Weil <sage@redhat.com>
Reviewed-by: Samuel Just <sjust@redhat.com>