LU-13055 changelog: use default mask if server has no mask

LU-13055 changelog: use default mask if server has no mask

When registering a new maskless user and server has no specific
mask set then effective mask to be set to DEFAULT value

Fixes: a15eb4f132 ("LU-13055 mdd: per-user changelog names and mask")
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: If799cb5cc29c60cce6ef6c987f2e493145e00e31
Reviewed-on: https://review.whamcloud.com/44404
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>