LU-17612 sec: return keyring errors to userspace

LU-17612 sec: return keyring errors to userspace

In current code, Linux keyring errors, when using GSS Kerberos,
are all masked under a generic ECONNREFUSED error. That makes
it hard to understand the root cause of the problem
for the I/O caller.

Update the code to propagate errors from request_key() up to
the application.

struct ptlrpc_cli_ctx * gss_sec_lookup_ctx_kr(...) is modified
to now returns a NULL pointer or -errval. This is tested by callers
and propagated. NULL values are still converted to ECONNREFUSED.

Lustre-change: https://review.whamcloud.com/54296
Lustre-commit: cd8625792f10d51fceca4717544ff8016609c3be

Test-Parameters: trivial
Test-Parameters: kerberos=true testlist=sanity-krb5
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Change-Id: I13792f141a961036bc9f7629a4a2db692e245c41
Signed-off-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54556
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>