LU-13343 gss: no sec flavor on loopback connection

LU-13343 gss: no sec flavor on loopback connection

When using a local client, i.e. a client mounted on a server node,
there is no benefit from a security standpoint to enforce an SSK or
KRB flavor, since the data does not go over the network.
So force the 'null' security flavor for connections on 0@lo,
independently of the currently defined srpc flavor.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: If25d69bb1e67735cb0544ca954e49175f7471248
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/46704
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>