Fix keyvaluestore fiemap bug

The result of fiemap is wrong and the offset get from
"StripObjectMap::file_to_extents" need to multiply by sequence number

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1912 from yuyuyu101/keyvaluestore-sparse-read-bug

Fix keyvaluestore fiemap bug

Reviewed-by: Sage Weil <sage@inktank.com>