SUNRPC: remove all refcounting of groupinfo from rpcauth_lookupcred

current_cred() can only be changed by 'current', and
cred->group_info is never changed.  If a new group_info is
needed, a new 'cred' is created.

Consequently it is always safe to access
   current_cred()->group_info

without taking any further references.
So drop the refcounting and the incorrect rcu_dereference().

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>