EX-7601 osc: rewrite compress_request

EX-7601 osc: rewrite compress_request

The existing version of compress_request can't handle
discontiguous RPCs.  Rewrite the logic to handle this
case properly.

This also implements kms handling.

If a write chunks ends at the known minimum size, we know
this write is after all other data in the file and so
there is no compressed data under it.  This means we can
compress this chunk.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I8a912d9e279d04c8ff07de39e63a1ec9b490d921
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53111
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>