EX-9192 csdc: Fix the upper mergeable chunk pointer

EX-9192 csdc: Fix the upper mergeable chunk pointer

If the full chunk is followed by un mergeable page, the upper
mergeable chunk pointer is occasionally set to this unmanageable page.
The chunk size is calculated wrongly then and the next condition
suggest not to compress this chunk, because its size is not equal to
the expected size.

The pointer should be moved to the first instruction after the
can_merge_pages().

Signed-off-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Change-Id: I09fedc770c8bbcac4864b32372a941da5e0c7ac3
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54814
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>