SUNRPC: Remove unnecessary memory barriers

The only check for RPC_TASK_RUNNING is the one in rpc_make_runnable(),
which happens under the same spin lock held when we call
rpc_clear_running().

Ditto, the last check for RPC_TASK_QUEUED in rpc_execute() is performed
under the same lock as the one held when we call rpc_clear_queued().

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>