LU-18497 gss: carry out creds prepare as user

LU-18497 gss: carry out creds prepare as user

Instead of forking a child process that switches to user identity to
get the credentials cache name, just carry out the whole credentials
preparation as the user, and switch back to original uid/gid in order
to proceed to ioctls, eliminating the need to map memory between
processes.

Test-Parameters: trivial
Test-Parameters: testgroup=review-dne-selinux-ssk-part-1
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Test-Parameters: kerberos=true testlist=sanity-krb5
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I7807263d71cd3fc8a9934cc4c4da8b497b845e6f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57192
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>