SUNRPC: simplify auth_unix.

1/ discard 'struct unx_cred'.  We don't need any data that
   is not already in 'struct rpc_cred'.
2/ Don't keep these creds in a hash table.  When a credential
   is needed, simply allocate it.  When not needed, discard it.
   This can easily be faster than performing a lookup on
   a shared hash table.
   As the lookup can happen during write-out, use a mempool
   to ensure forward progress.
   This means that we cannot compare two credentials for
   equality by comparing the pointers, but we never do that anyway.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>