LU-14462 gss: fix support for namespace in lgss_keyring

LU-14462 gss: fix support for namespace in lgss_keyring

Fix the way lgss_keyring handles different mount namespaces,
so that we do not try to bind to a namespace that does not exist.

Fixes: 94c44c62de ("LU-7845 gss: support namespace in lgss_keyring")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ia5a5213399decc683d5e9401b6594e7fe579123f
Reviewed-on: https://review.whamcloud.com/41716
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>