SUNRPC: Fix a lock recursion in the auth_gss downcall

 When we look up a new cred in the auth_gss downcall so that we can stuff
 the credcache, we do not want that lookup to queue up an upcall in order
 to initialise it. To do an upcall here not only redundant, but since we
 are already holding the inode->i_mutex, it will trigger a lock recursion.

 This patch allows rpcauth cache searches to indicate that they can cope
 with uninitialised credentials.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>