svc: Move the authinfo cache to svc_xprt.

Move the authinfo cache to svc_xprt. This allows both the TCP and RDMA
transports to share this logic. A flag bit is used to determine if
auth information is to be cached or not. Previously, this code looked
at the transport protocol.

I've also changed the spin_lock/unlock logic so that a lock is not taken for
transports that are not caching auth info.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Acked-by: Neil Brown <neilb@suse.de>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Greg Banks <gnb@sgi.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>