SUNRPC: Fix a credential refcount leak

All callers of __rpc_clone_client() pass in a value for args->cred,
meaning that the credential gets assigned and referenced in
the call to rpc_new_client().

Reported-by: Ido Schimmel <idosch@idosch.org>
Fixes: 79caa5fa ("SUNRPC: Cache cred of process creating the rpc_client")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>