SUNRPC: remove BUG_ON from rpc_call_sync

Use WARN_ON_ONCE instead of calling BUG_ON and return -EINVAL when
RPC_TASK_ASYNC flag is passed to rpc_call_sync.

Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>