LU-18633 gss: carry out creds negotiation as user

LU-18633 gss: carry out creds negotiation as user

Instead of switching back to root id right after we did credentials
preparation, just do the whole credentials negotiation as the user.
We just need root privileges to proceed to ioctls that exchange data
with kernel space.

sanity-krb5 test_11 is added to exercise this.

Fixes: 6791fbc530 ("LU-18497 gss: carry out creds prepare as user")
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: I93dd367abadca077e61c1910638337a2d80996c8
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57752
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>