mount: use sec= that was specified on the command line

When older servers return RPC_AUTH_NULL, it means the
rpc creds will be ignored. In that case use the sec=
that was specified instead of setting sec=null

Fixes Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1112983


Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>