LU-14553 changelog: eliminate mdd_changelog_clear warning

LU-14553 changelog: eliminate mdd_changelog_clear warning

When handling a changelog_clear request, the user may specify a
range of indices which do not exist.  Similarly, the user may
specify a changelog user which does not exist.  Neither indicates
a problem within Lustre that justifies a a console warning.

Change those cases to CDEBUG.

Lustre-change: https://review.whamcloud.com/43125
Lustre-commit: 6b183927e19715d093c80a35ebc42a1cda5e70e2

Test-Parameters: trivial
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: I64bab12ef4978c4bf7139f5f36a39f9b109616fb
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46758
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>