LU-16734 gss: fix lookup_user_key() bug

LU-16734 gss: fix lookup_user_key() bug

With more recent kernels, like on Ubuntu 22.04, trying to
delete some keyring resources trigger a kernel warning message
and cleaning is not successful, leading to stuck resources
and warning messages being regularly printed.

This is because Linux 5.8, in commit 8c0637e, introduced an API
change for lookup_user_key() that was not taken in account.

Update the lookup_user_key() call from _user_key() to fix it.

Change-Id: I34ef4dac3f56cbb4aac6bc5a3bad36feb66b8675
Signed-off-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50623
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Jonathan Calmels <jcalmels@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>