LU-16641 tests: 12b: unlink recently created files

LU-16641 tests: 12b: unlink recently created files

It is possible that createmany hasn't created requested
number of files on mdt0. So remove only the number of
successfully created files instead of requested.

Fix error_ignore to avoid default error behaviour.

Fixes: 25896b8b88 ("LU-16641 tests: fix sanity-quota_12b")
Signed-off-by: Sergey Cheremencev <scherementsev@ddn.com>
Change-Id: Ic6d5a02295c73fbed0773408c67a47850dee1f80
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57929
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>