sunrpc: add a new "stringify_acceptor" rpc_credop

...and add an new rpc_auth function to call it when it exists. This
is only applicable for AUTH_GSS mechanisms, so we only specify this
for those sorts of credentials.

Signed-off-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

diff --cc include/linux/sunrpc/auth.h
Simple merge

diff --cc net/sunrpc/auth.c
Simple merge

diff --cc net/sunrpc/auth_gss/auth_gss.c
Simple merge