LU-16532 sec: session key bad keyring

LU-16532 sec: session key bad keyring

At initialization, the session key created for GSS context is linked
to a keyring from userpace, and then unlinked from kernelspace if it
is for root, as we want to share it across all root sessions.
Sometimes initialization fails (expired token, unresponsive server,
etc.) so the key cannot be unlinked. Survive this use case gracefully.

Lustre-change: https://review.whamcloud.com/49909
Lustre-commit: 1f034cf610deeb6b070338a9d005b75ce964f945

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I8ad2afa0e51e50640620e36211e5db1253d85e08
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50002
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>