Btrfs: fix incorrect block_len in merge_extent_mapping

Btrfs: fix incorrect block_len in merge_extent_mapping

Orabug: 27446668

%block_len could be checked on deciding if two em are mergable.

merge_extent_mapping() has only added the front pad if the front part
of em gets truncated, but it's possible that the end part gets
truncated.

For both compressed extent and inline extent, em->block_len is not
adjusted accordingly, while for regular extent, em->block_len always
equals to em->len, hence this sets em->block_len with em->len.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>