sunrpc/auth: allow lockless (rcu) lookup of credential cache.

The new flag RPCAUTH_LOOKUP_RCU to credential lookup avoids locking,
does not take a reference on the returned credential, and returns
-ECHILD if a simple lookup was not possible.

The returned value can only be used within an rcu_read_lock protected
region.

The main user of this is the new rpc_lookup_cred_nonblock() which
returns a pointer to the current credential which is only rcu-safe (no
ref-count held), and might return -ECHILD if allocation was required.

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