LU-14263 gss: unlink revoked key

LU-14263 gss: unlink revoked key

When a GSS context is destroyed, it is unbound from its key, marking
the key as revoked.
The key also needs to be unlinked from the session keyring. This way,
a subsequent context initialization will manage to create a new valid
key and link it to the keyring.

Similarly, add a new '-r' flag to 'lfs flushctx', in order to reap the
revoked keys from the keyring when flushing the GSS context.

Test-Parameters: trivial
Test-Parameters: clientdistro=el7.9 testgroup=review-dne-ssk
Test-Parameters: clientdistro=el8.3 testgroup=review-dne-ssk
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ida4b4ea53202c1f40ad93816fb4ec96fec2bf8bc
Reviewed-on: https://review.whamcloud.com/41047
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>