LU-17566 mdt: improve new_init_ucred() for refactoring

LU-17566 mdt: improve new_init_ucred() for refactoring

In order to merge new_init_ucred() and old_init_ucred()
code eventually, move new_init_ucred() code around
for it to look even closer to old_init_ucred().

- Fill generic ucred fields at the beginning (similar to
what old_init_ucred() is doing.
- Move code for the bottom part to be closer to
old_init_ucred_common().

This code path is not used on most of lustre deployments,
so I'm enabling kerberos testing to ensure some tests
will go through this code path.

Test-Parameters: kerberos=true testlist=sanity-krb5

Change-Id: I113fca6a104c1db66d9e0defd6fd91e378d7208c
Signed-off-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55025
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>