EX-7601 ofd: add obd level compression lib

EX-7601 ofd: add obd level compression lib

Some compression functions will be used by several areas of
of Lustre, so they need to be in obdclass.

This moves merge_chunk and unmerge_chunk there and adds the
ability for them to merge lnbs.  This is used in a future
patch.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: If4a318119bb7685e41adb9f3b31a66074031e6ac
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52938
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>